Reprinted from CIRTULATION Vol. XX, No. 6 December, 1969 Printed in U.S.A. Relationship Between Plasma and Extracellular Fluid Volume Depletion and the Antihypertensive Effect of Chlorothiazide By ILSE M. WILSON, M.D., AND EDWARD D. FREIS, M.D. With the technical ass&arm? of Ma,ry J. Taylor The importance of plasma and extracellular fluid volumes in the mechanism of the anti- hypertensive effect of chlorothiazide is disputed. The present investigation indicates that the lowering of blood pressure is accompanied by reductions in plasma and extracellular fluid volumes and in body weight. Furthermore, re-expansion of plasma volume with salt-free dextran reverses the antihypertensive effect. However, since gradual reaccumu- lation of extracellular fluid occurs during 1 year of continuous treatment, the late anti- hypertensive effects of chlorothiaxide cannot be explained by the volume-depletion mechanism. T HE tliscovrry of the antihypertensive action of chlorothiazide'z 2 raised cer- tain questions as to its mechanism of action. These questions included (1) whether the hypotensive response was caused by salt de- pletion or by some independent factor ; (2) if produced by salt loss, by what mechanism it reduced blood pressure ; (3) the factors that lead to increased reactivity to other antihy- pertensive agents ; and (4) the reason for the moderate antihypertensive action of the drug when used aloue in hypertensive pa- tients alld the absence of such activity in normotensive subjects.`iv 4 Complete answers to these questions cannot yet be given. This report concerns an attempt t,o elucidate some of thr factors involved. MATERIALS AND METI~~D~ Twenty male hypertensive patients with no symptoms or signs of congestive heart failure or edema were hospitalized and placed on a stand- ard diet containing 1.5 Gm. of salt per day plus a supplement of 3 Gm. of salt in tablet form. This supplied a sodium intake of approximately 75 mEq. per day. Most of these patients were From the Veterans Administration Hospital and t,he Cardiovascular Research Laboratory, Department of Medicine, Georgetown University School of Medi- cine, Washington, I>. C. Supported in part by U. S. Public Health Service grant H-720 (National Heart Institute) and by a research grant from Merck, Sharp and Dohme. under treatment with other antihypertensive agents (table l), which were continued during the entire period of study. A a-day period for acclimatiza- tion to the diet was instituted in order to obtair stabilization of body weight and electrolyte excre- tions as well as average basal blood pressure readings. Determinations of plasma and extra- rellular volumes, serum electrolyte concentrations, and serum bicarbonate were carried out on thr morning of the fifth day. Each patient was then given 500 mg. of chlorothiazide twice daily, and the studies were repeated after a period of 3 to 8 (average 6) days. Tn 11 patients carefully screened for reliability and conscientiousness in regard to taking thei, Itledications, studies were repeated at approxi- mately 6-month intervals for a period of 18 months. Seven of these patients were hospitalized and placed on the controlled salt intake for t' to 3 days prior to repeating the determinations. while the remaining 4 were hospitalized for 1 day only because of thrir inability to take time out from work. Thus, dietary intake of salt cannot be regarded as well controlled in the long-term studies. At approximately 6 nlonths following initiation of treatment the determinations of plasma and extracellular volumes in `i patients were repeated. Chlorothiazide therapy was then discontinued for 1 week, after which another series of determinations were carried out. The patients remained hospitalized throughout this lattm period. Plasma volume was determined in the fasting subject after one half hour of rest in the supine position with the Evans blue dye method of Gibson and Evans" adapted and modified for use in the Coleman spectrophotometer,@s 7 3 specimens being 1.028 Circrlation, Vohne XX, December 1959 FT~I:ID VOLUME DEPLETIOS 1029 (`.E. 46 .J .\Vr. 4i s.xr. GO r\..l. 46 .I .Er. 43 .l.HlI. Ii0 11.K. 10 .I.%. .5 7 A.W. Iii S.K. 50 .I .We 66 ( `.H. `I'. IT. .I .( `, c.ri. .\T.l'. ci.1:. .I .I ). \I.(`. s. I+:. RG titi 47 5s 4ii Mean s. I). +%I +4.4 +5.0 + ..5 +3.0 j-%2 +1.i +1.0 - .4 +1.8 +4.0 + .3 . 0 +J.4 + .;i +I.1 $4.4 +4.2 +1.-r + .-I +L!.0 --II:! -.iOl -7s:: -132 -319 -.iAX - 66 --t'!lG - 55 -460 --2G.j 59s -2. 4 ::ci:: .:i --43/10 1.1 29 -3.x -172 + .4 4.1; 6.4 --04/-I,j" +s.:!" 11/7.5 4.9 xp va111e less than .OOl. 11stv1 tit 10, 15, :ttttl 20 tttitttttcs i'or cl~~tertttittitig tlyc tlilutiott. `I'he lietttatwrit value wits detertrritwtl frotn the avertage of 6 stittlples drawn during the various experitnental procedures. Eighteen ntilli- liters of 5 per cent sodium thiocyanate" were in- jwtrd usudly at the end of the plasnia volutw tlt'tc,t.tttitr;ttic)tis and s:tttlplrs WeI'? drawn at 2 ilttd :i I10urs for tletcrntitxttion of thineyanate spnw :twortlittg 1.0 tlte tttetltocl of Cranclall nnd Antler- hl)llh :td;tptd to thr Coletnnn sl.`e~trophototttrtrr. I~;itliosulI':ttc sp:tw was dc~tcmtiinrd with SC::- I:llwld srlll';rtc~ I,y tlw ltlrtllod of \valsrr." sodillttl :lttd potit>>ittttt \vcw dctrrttiined with a flninr 1030 WILXOS, FREIS I CHANGE IN SCN T2 SPACE - LITERS . . I I . I o ? I4 . were analyed for total sodium, chloride, and potassiuul, and the averap~ was taken as the con- trol level of rxcrrtion. The cunudativa elevations ahovv this lcq~~l tlurin~ the lirst 3 to 1 days a.fter c~hlorothiazidt~ wvw tllvtk uwtl to cdinl:itc~ elf&w- 1ytr losst%. The effect of restoratiott of tlifb plasma volulnc was detennilled in 7 patients who exhibited sig- nificant llypot~ensive responses to chlorothiazide. Inch patient rweivcd X0 1111. of 6 prr crnt des- tran in normal saline; and after several days the procedure was repeated with substitution of a sinlilar volut~~e of dextran in 5 per cent alucosr in water. The infusion rate was approximately 25 ml. per minute. Blood pressure and heart rate were detertninrd repeatedly before and through- out the procedure and blood for hrnlatoerit detrr- Anations was drawn innnrdiately preceding and followin=; rach infusion. REs1iI,Ts Following 3 to S (mean 6) days of chloro- thiazide therapy the plasma volume showed some reduction in all of 20 nonedematous hypertensive patients (table 1). The loss of plasma volume averaged 338 2 223 ml. and varied in different patients from insignifi- cant falls of less than `i0 ml. to rather marked losses above 700 ml. The hematocrit reflected the plasma volume change in an approximate way by exhibiting slight to moderate eleva- tion. However, there was no close quantita- tive relationship between hematocrit and plasma volume changes. T.\BLE 2.-Lo.sscs nflcr CkloroHiiazide Change after Chloro- Urinary losses: chlorothinzide thiazide cumulative negative Thiocyanate - treatment balance (mF.q.) spice wt. Patient (days) Na Cl K CL.) (Kg.) s. EC 3 "24 392 "09 --A6 -5.4 J. Bu. 4 301 489 181 -1.9 -2.T s. 31. 4 290 560 168 -2.4 -3.3 J. Z. 3 162 135 154 -3.9 -2.2 J. T. 4 216 268 4 - .7 -2.2 E. W. 3 349 510 222 -3.4 -2.7 - Average 3.5 25$* 392" 156.3" 2.8" 3.0" s. I). 68 163 71 1.4 1.2 "p value less than .005. The thiocyanate space declined by more than 0.5 L. in all except 5 of the 20 cases (table 1). For the group as a whole the mean reduction was 2.1 5 1.75 L. Although the reductions of both plasma and available fluid spaces were significant (p < .OOl), there was 110 consistent relationship between the degree of plasma volume and available fluid changes. Body weight decreased in 16 of the 20 patients (mean 1.8 * 1.84 Kg.). The change in body weight was significant at the .OOl level. There appeared to be a rough correla- tion between the extent of change in body weight and in thiocyanate space (fig. 1). The blood pressure fell in all patients, the average change bring -%I/15 mm. Hg. The reduction occurred during the first 48 hours following institution of chlorothiazide. There was no quantitative correlation between the plasma volume and arterial pressure changes. The cumulatiw losses of body electrolytes (cumulative negative balance) by the end of the third to fourth day of chlorothiazide treat- ment averaged 25i 2 68 mEq. of sodium, 392 2 163 mEq. of chloride, and 1.56 * 71 mEq. of potassium in 6 patients studied (table 2). All of these patients exhibited significant re- ductions in body weight and thiocyanate space. The correlation between the extent of body weight reduction and electrolyte losses was poor in this small series. However, the accuracy of 24hour urine collections cannot PLUIU VOLUME I)El'LETIOA' 1031 Patient Chloro- thiazide treatment ( mos. ) Con ttd B.P. (mm.Hg) After cbloro- thiazide chanae mean B.P.* (%"o) Following dextran infusion In 5 gel- cent In normal alueose saline chanpe chanae ___- Mean B.P. Hematocrit Mean 1S.P. Hematocrit (%) (%) ( % ) (76) J.H.f 1.0 150/110 -16.2 J.C.y .5 180/110 09 g -I&. C.S.$ 3.0 150/110 -13.0 li.C.$ 1.0 180/108 ---30.4 W.F.? 1.0 140/100 -11.3 N.B.$ 1.0 145/95 -12.5 .1.x.t 2.0 148/96 -10.9 Mean 1.4 155/104 -16.7~ +10.0 +18.4 +10.5 +t'S.l +14.0 j19.0 + 8.4 ._____.- +15.4s - 3.2 -11.4 ---1'7.1 - 9.1 -1::.ti -- 8.8 -- 5.0 - 9.0$ + 9.0 - 5.8 +L?o.u --120 +1o.u -14.8 +X3.7 -14.G +1x2 - 9.8 +17.0 ~ 6.0 - +15.4# - 9.8# S.D. 8.8 G.!l 3.8 5.8 3.3 *` ` Rknn" blood pressure equals (systolic + diast,olic) /2. j'0n no other antihypertensive drugs. be regarded as completely reliable, even in hospitalized patients. The changes in the serum cottceittrations of sodium and chloride were insignificant, but there was a small but significant decrease in potassium (table 1). The serum bicarbonate determined in 4 pa- tients shomed essentially no change. ckxtran in 5 lwr cent glucose ill water. In 7 patients a significaltt eleratioti of blood pres- sure was observed ranging betwell 8 to 2& mm. Hg (mean + 15.0 2 7.0 111111. Hg, (p < .005) The percentage fall of hemato- crit awraped 9.0 * 3.8. Acute Ikstoratiou of Plnsnaa Volume thiazide The effect of re-expansion of plasma Whereas the reductions in plasma volume, rolume on blood pressure was determined in thiocyanate space, and body weight mere 7 patients who had been under continuous significant during short-term therap)- with treatment with chlorothiazide for periods chlorothiazide~ this was not t,he ease follow- railging from 2 weeks to 3 months (mean ing long-term treatment. In 11 patients mhen 1.4 months). These patients had exhibited these parameters were measured at the elrtl reductions of " meau " (systolic + diastolic) /2 of 6 months, old>- the rcdnctiotl in thio- blood pressure averaging 17.0 c `1.0 mm. Hg vyauate spacae rcmainec1 si&?cant (table 4). (table 3). The administration of G per cent At the end of 1 year the change irt thiocyanatc dcxtralt in isotonic salirie in 6 of these pa- space was no longer significant at the .05 tients resulted in an immediate elevation of level. Despite this trend, hoverer, the blood blood pressure averaging 15.0 * 6.0 mm. Hg. pressure changes, although not as marked at The mean percentage fall in the hematocrit the end of 1 year as in the early stages of was 9.8 2 3.3 during the dextran infusion. therapy, still showed significant rcduet,ions. Tn order to determine whether sodium ion In 9 patients after 1 to 3 months (average was important in the reversal of the anti- l.S months) the rxtraeellular flnid volume hypertensive effect the infusions were re- was estimated by means of 2 indicators (table peated after several days with 6 per cent 5). thioqwrate and radiosulfate simultane- WILSON, FREIH TABLE 4.-Changes Initially after Xc and l'weke Months --- ~~~- . __.. Plasma volume Blood pressure Thiocyanate space Hody weight (ml.) (mm. Hg) CL.) (Ka.) _____..~~ -~ -~- Patient 1 wk. 6 mo. 12 mo. 1 wk. 6 mo. 12 mo. 1 wk. 6 mo. 12 mo. 1 wk. 6 mo. 12 mn. -~__ J .We. -265 - 22 +165 --ao/-1s -3o/--15 O/--8 -5.8 -3.9 - .i -4.6 -1.1 -2.8 J.Wr. --Xl - 49 --112 -26/-16 -2?/-20 -~ 7/O - .9 -2.2 - .2 - .9 -6.0 -6.1 C.8 / . -741 -201 -225 -14/-20 -2o/-20 -3oi'--30 -4.0 -3.2 --I.? -1.4 -8.0 --4.9 N.M. -783 + 74 f132 -17/--F -2o/-19 --35/-14 -2.4 ---2.4 $1.2 -3.3 + 2 -1.0 T.11. -598 +110 -168 -4o/-2.5 -3o/-20 -w-10 -2.4 - .4 - .i -3.0 +7.7 f(i.7 s.13. -596 + 58 - 19 --05/-E -N-20 -20/--o:: -4.6 -4.7 + .9 -5.1 -6.3 +5.i J .(:. -363 -3'3 -402 -2()/-2" --.30/f-20 30/-5 - .5 - .5 -.I +.r! + .9 +4.9 J.R1. -319 -248 -534 -3o/-20 -28/-15 -10/o + .1 -2.1 -2.1 +".o +4.4 +1.9 J .Bn. -568 -164 -336 -E-l0 -E--PO -lo/-l5 -1.9 - .8 - .6 -2.7 + .8 -1.4 C.P. -177 - 51 - 51 -so/-30 -3o/-20 -u/-20 -2.1 -1.1 -1.5 -2.3 -4.3 ---7.0 H.K. - 66 - 75 - 56 --8o/-20 --2oj--20 --`o/-15 - .5 - .9 - .7 -1.2 - .I + .L ~~ 31ean -452" - 80 -1557 --37/m* -24/x9" -n3/13* --?.3$ -2.01 - .6 ".o$ -1 .o - .4 s. I). "s:: 128 212 11.6/6.8 5.5/t! 10.5/" d. 4 I .8 1.5 .9 2. I 4.6 4.7 ~-___~~ *p value less tlmn .OOl. tp r:llne leas than .05. $p vn111c less thm .Ol. Duration Thioeyanate s~aee SWabeled sulfate space Thiocyanate $y; treatment CL.) (L.) change ehanae Patient ( mos. ) Control After dup. Chanae - Control After drug Change (5%) (76) J.W. N.&L A.J. T.II. R.E. .I .Bu. .I .14r. (1.P. IT. Ii. 2 3 `, 1 1 6, ,- -.., 1 1 1 "5.4 19.8 24.0 25.6 24.5 19.11 00 1 --. 18.9 17.1 23.2 17.4 21.; 25.0 "1.1 1X." 18.9 1 ti.3 16.8 i, 0 --.- -2.4 -2.3 --- .(i -3.4 -1.X --3.5 ---2.6 - 2 19.9 14.1 17.7 19.6 18.3 14.1 lS.li 15.8 13.2 15 7 t . 13.6 Ii.3 19.6 1 (i.0 13.2 15.8 14.1 12.4 -4.2 -0.5 ~- .4 -0 -2.3 -1.2 -2.8 -1.7 - .8 -1 1 .r, -12 --I 0 0 -14 -11 -I6 -14 -2 Rlean 1 xi 21.8 19.9 -2.1.5 lG.8 15.3 -1.5 -1 (,.3X - -9.at RI). I.1 I .:1 5 7.3 *p wrloe less than .OOl. tp value less than .05. ously, as ill1 additional cehec*k 011 t,lle validity of the change demonstrated with thiocyalmte alone. Considerably more variation was ob- served with radioslilfate as the indicator. ITowevrr, the averagr vhangc for the group as a whole was similar with either thiocyanate or P-labeled sulfate and the reduction in estimated extracellular space was still sig- nificant with either method. Chlorothiazide was withdrawn for a l-week period in 8 patients after 3 to 7 (average .5.4) mont,hs of contiuous treatment in order to evaluate the changes that might occur fol- lowilig tliwontitiuation of long-term therapy (table 6). There was a prompt elevation of plasma volume averaging 241 -C 100 ml., a rise in blood pressure (mean 23/15 * 8/9 mm. Hg), and a gain in body weight aver- aging 1.6 * 1.3 Kg. All these changes mere signific'ant. at the .01 level or less. The in- FLUID VOLUME DEPLETION T.IBT,E 6.-Changes Following Disconlint~ation of ChZorothGazide Plasma volume Blood pressure Thiocyanate space Body weight Duration (ml.) (mm. Hg) CL.1 (Kg.) tryat~e,nt Af;e; Repeat Ga$x .4fkr Repeat Gain or After Repeat Gain or Patient m . . control chlor. control 1OSS chlor. control 1OSS after Repeat "a&or chlor. control J.We. J.Wr. C.S. T.H. S.E. J.C. J.Rr. H.K. 4 3720 3827 +107 7 3717 4106 +389 i 3537 3871 f"34 7 3943 4152 +209 7 3183 3525 +349 5 3340 3663 +323 3 3200 3350 +150 3 2931 3106 +175 170/l 00 f00/190 +30/-00 151/103 180/130 +29/27 I P8/87 150/110 +2?/?3 153/l 00 160/110 + 7/19 160/170 180/l 25 +"0/15 170/100 n00/110 +30/10 140/100 165/100 +25/o 130/90 150/110 +00/20 33. 06.9 56.4 25.4 19.6 20.7 54.7 25.6 "4.2 25.6 02.8 23.0 18.9 SP.4 16.4 17.2 +2.1 -1 .o +1.1 + .g +1.4 + 2 +3.5 + .8 S.i..? 85.3 1n3.3 I04 73.0 74.3 90 94.3 71.5 70 d. 7 95.4 99.5 80.3 83.0 7n 70.7 0 + .i +1.3 +2.3 +1.2 +4.1 +2.7 + .7 Average 5.4 3446 3700 +241* 150/99 173/144 +?3"/15? 21.5 23.4 +1.1 83.8 85.2 +l.Sf S.D. 100.4 7.8,`8.8 1.4 1.3 *p value less than .OOl. tp value less than .05. crease in thiocyanate space averaged 1.1 `- 1.4 L. The latter change was of questionable significance (p < .06). As compared to pretreatment values it was of interest that following discontinuation of the drug plasma volume increased 120 t 18 ml. and thiocyanate space rose to 0.5 -I 0.3 L. above the original, untrrated level. These slight ovrrshoots werr not accompanied by parallel behavior of the blood pressure, t.he latter rcmainillg -4/6 f 7/7 mm. Hg below the pretreatment level. The cha.nges in serum electrolyte concen- trations and in serum bicarbonate were dc- termined after 1 year of chlorothiazidr treatment in 11 patients (table 7). Serum sodium concentrations were essentially nn- changed. Serum chloride levels decreascad slightly in most patients bnt increased slightly in 2, the average change being 3.4 -I- 6.8 mEq. There was a significant decline in serum po- tassium concentrations. The rn(`an reduction was 1.1 f 0.6 mEq. (p .OOl). The serum bi- c.arbonatP remained unchanged. DISCUSSION In the present observations the reduction of blood pressure paralleled the saluresis and did not precede or follow it. The saluretic effect was essentially complete in 48 hours. The quest,ion arises as to the source of the sodium depletion, specifically as to whether it represented reductions in int,racellular stores or came from the extracellular fluid. The maintenance of normal serum sodium levels indicated that the drug did not ex- tract sodium from the ext,racellular water. Rowever, the extent of the depletion of ex- tracellular fluid volume indicated that the majority if not all of the sodium removed could be accounted for by an excaretion of isotonic, extracellular fluid. This is in keep- ing with the observations on other saluretic agents such as mercurials, which produce pri- marily a rednction of ext,racellular fluid volllIlle.l" Since the plasma volnnw is in equilibrium with the interstitial fluids, it also shared in the general redurtion of extracellular fluid space. However, the relationship between change in extraeellular and plasma volumes is only approximate. I3 Because of the failure to find any evidence for significant cellular losses of sodium, the question arose whether the antihypertensive effect was related simply to the decrease in total circulating blood volume and to a reduction in tissue pressure secondary to t,he extracellular fluid loss. Such an interpretation is supported by the observation of Crosley and his associateslJ that right heart pressures and cardiac output are reduced by chlorothiazide. Preliminary 1034 WILSOK, FR,EIS TABIZ T.-Changes Following One Year of Treatment with Chlorothiazide - Patient Na Change in mEq. Cl K CO? J.Wc. -3 -4 - .8 -1. J .Wr. --1 -I? - .3 -1.6 C.S. 0 -9 - .6 -2.0 S.M. -1 -9 - .6 +4.8 T.TI. $2 -11 -1.1 +x1 S.E. 0 -8 -1.5 +3.`3 J.C. -4 -1 - .4 +s.o .J.Uu. -5 +g -1.2 -3.0 J.Rr. -1 -3 - .4 -1.0 C.P. -4 +5 -2.1 -4.0 1I.K. 0 -5 -1.0 +1.0 ~___ Mean -1.5 - 4.4" - .93t + .,o S.D. `, r -.J 6.8 .6 2.9 *p vahe less than .05. tp value less than .OOl. results ill this laboratory tend to confirm these observations. Dustan aud her co-mork- crslZ also found a reduction iu cardiac out- put following chlorothiazide. If the antihypertensive effects of chloro- thiazide mere due to a change in intracellular sodium coucentratiou producing a decrease either in arteriolar tow or a dehydration Of "waterlogged " arterioleqi" the hemodg- llamic ctI'ects should be reflected iu a reduc- tion of total peripheral resistance rather thaii a fall iii cardiac filliiig pressures ant1 output. The latter fiutlings, however, are readily c~xplainetl by the dwrcasrs in plasma atrtl extrac~rlluliir fluid volumes. It is of in- tcrrst that the normotcnsive individual com- pensates so that the basal blood pressure is Ilot lowered, whereas this compensation seems to be deficieut in hypertensive patients."* 4 Whether such failure of homeostasis is due to a decreased responsiveness of the barore- ccptors or to other factors cannot be deter- mined on the basis of present evidence. The importance of the plasma volume change in the mechanism of the antihyper- tensive effect was indicated by the fact that restoration of plasma volume either with or without any replenishment of sodium re- stored the blood pressure to essentially pre- treatrnent~ levels. The lack of a yuantitativc relationship between t,he decrease in plasma volume and the fall of blood pressure can be explained on the basis of variations in the activity of compensatory mechanisms in- cluding the baroreceptor reflexes, and in- trinsic vascular distensibility in different individuals. This variability was further enhanced by the fact that some of the patients mere taking ganglion-blocking drugs and others were not. These cotmlusions on the importance of oligemia iu the mechanism of the antihyper- tensive action of chlorothiazide confirm our initial find&$ and are similar to those ex- pressed by Dustan, Tapia, and associates.`", ii They propose that the oligemia enhances rasomotor "tone" which in turn makes the patient more responsive to ganglion-blocking agents. However, the observation that the hypertensive patient often shows some reduc- tion of blood pressure with chlorothiazide alone whereas the normotensive subject does not33 4 suggests that the compensatory mechanisms for plasma volume depletion often are in- adeyuate in hypertension. Also, the decreased responsiveness to norepinephrine following chlorothiazide'", lQ in normot,ensive subjects is imt readily explained on the basis of iti- creased vasomotor tone alone, and suggests that reactivity is dependent to sorne extent ou the degree of filling of the vascular system. The fa,ilure to obsrrvc a significant reduc- tion of plasma volume after 6 to 12 months of treatment reflects either tolrrancc to the saluretic effects of the drug or the establish- ment of compensatory mechanisms for res- toration of homeostasis. Compensatory mech- anisms become active after the first 48 hours of treatment when the output of sodium comes back into balance with the intake. However, in the first month of treatment the depletion achieved during the initial salure- sis is maiutained and only gradually, there- after, is the plasma volume deficit made up. Other investigators have postulated that chlorothiazide may have antihypertensive effects additional to its saluretic action.* This FLUID VOLUME DEPLETIOJJ 1035 is based on the observation that the blood pressure remaius below pretreatment levels after the body weight has beeli restored. It should be pointed out, however, that disap- pearance of acute drug effects without return to pretreatment levels of blood pressure is not unique for chlorothiazide. For example, the acute heniodynamic effects of hydralazine, characterized by taehycardia and palpitation reflecting ail increase ill cardiac: output, geu- erally disappear with long-term treatment. Similarly, the manifestations of ganglioiiic blockade such as impaired visual accommoda- tion, dry mouth, and postural hypotension often diminish with loiig-continued therapy even though basal arterial pressure remains below the pret,reatment lcvcl. The observations of Perry and SchroederzO indicate that vigor- ous treatment to obtain a continuous and pro- longed reduction of arterial pressure, often modifies the severity of the hypertension, so that less intensive or no further treatment is required. Thus, after long-term therapy many factors, such as tolerance, compensa- tory reactions, and modification of the basal level of blood pressure, come into play to obscure t,he initial relationship between drug action and antihypertensive effect. It does not seem possible to draw valid conclusions concerning the antihypertensive activity of a drug at this late stage, especially if the effects of drug withdrawal are not deter- mined. SIJMMARY AND CONCLUSIONS Plasma and extracellular fluid volumes, serum electrolyte concentrations, arterial pressure, body weight, and electrolyte excre- tions were determined in hypertensive pa- Cents treated with chlorothiazide. Plasma aud extracellular fluid volumes tlccreased promptly during the early phases of treatmcirt. This was accompanied by a re- tlnctioll of :I ierial pressure and body weight. Sodium losses could be accounted for on the basis of extracellular fluid volume depletion. Restoration of plasma volume either with or without partial replenishment of sodium reversed the antihypertensive effect of chloro- thiazide. Withdrawal of chlorothiazide after 3 to i months of treatment was followed by an elevation of plasnia volume usually to levels slightly above the control. Blood pres- sure rose to levels slightly below the control. Significant reductiolis of plasma volume and body weight were not found after 6 months and of extracellular fluid volume after 12 months of therapy even though the blood pressure remained reduced. However, the latter may not be a valid criterion of drug activity following long-term modification of the blood pressure level. The only significant change in serum electrolyte concentration was a reductiou in serum potassium. Serum bicarbonate levels were not altered. On the basis of this and other evidence tliscussed it is suggested that the decrease in plasma volume is an important, factor pro- ducing the init,ial antihypertensive effect. Re- duction in tissue pressure secondary to extra- cellular fluid volume depletion also could contribute to this response. SUMMARIO IN TNTERLINGUA Le volumines de1 plasma e de1 liquid0 extracellular, le coinentrationes de1 electro- lytos in le sero. le tension arterial, le peso corporee, e le cscretion tlr rlectrolytos esseva tleterminat,e in patientes hypertensive sub- tractamento con chlorothiuzido. I,e volumines de plasma e de liquido extra- cellular descendeva promptemeilto durante Ir phases initial de1 tractamento. Isto esseva accompaniate per un reduction de1 tension arterial e de1 peso corporee. L.c perditas de natrium esseva explicabile super le base de1 tlepletion de1 volumine de liquido extracellu- lar. Tie restauration de1 plasma, tailto coil coma etiam sin le restitution partial de nat- riurn reverteva le effect0 antihypertensive dc chlorothiazide. Tie privation de chlorothiazido post 3 a 7 menses de tractamento esseva so- quite per un elevation de1 volumine de1 plas- rna, usualmente usque a uivellos levenientc supra le nivellos de controlo. T,e tension de1 sanguine montava usque a iGvellos levenieute infra le nivellos dc c:ont.rolo. 1036 Signifkati\-c rrductioncs dcl volumitie de1 plasma e del pcho corporee 11011 esseva coil- st.atatc ~iosf 6 liieilses dc therapia, c! similc- ttlentc HIIIIC sigllificativc reductiones de1 vol- imine clel licjiiido extracellular esseva cou- statatc post 12 mcllsea de thcrapia ben que lc tcusiou de sauguiue remaneva reducite. Tatneu, iste ultime facto es possibilemente iltralitle vo111e criteria de1 activitate de1 droga post LIII proloiigate modificatiou de1 nivello tlel teiisioii de sanguine. Le sol sigiiificative alteration iu lc concentration de electrolytes in le sero css(`va 1111 reduction de1 content0 de Icalilun. I :(A tlivellos dc bicarbouato in le sero Lieu esseva alterate. Super 1~1 base de iste e altere observationes tliwutite in lc presente reporto, le these es fotmulatc~ q11c le reduction de1 volumiue de1 Illitslllil OS 1111 inlportaute factor in le produc- t ioii tlvl cfY~vA0 aiitihypertemive iuitial. Le rcvliwt~ioil (1~1 prcssioii de1 histos quc sequc In dr[llrtiou de1 volmnille cle liquid0 exkacel- llllar (1s etiam possibilemeute uu contributor it illc WSpOlls~. RFFE'HESCES 2 i ). 1. lh6is, K. I)., .\iv~ W~sos, I. Il.: Potenti:~t- ing effect of chlorothiazide (Diuril) in coiu- binatiou wit,h antihypertrnsirr agents. Med. :lnu. District of Colultlhia 26 : 36S, 1957. L'. IIOLL.\NDER, ITT., AXD ~~"ITXINS, R. Ii'. : Cl~lorotl~i:~zidc. ,\ new type of drug for trratuwnt of Ii\-pertension. Boston U. Ned. Quart. 8: 69, 19Si. 3. FREIS, E. D., WANKO, il.. WILSON, I. Al., AND PAHHISII, d. E.: Chlorothiazide in hy- pertensive ant1 norillotrnsive patients. Ann. New I-otk Acad. SC. 71: 450, 19%. -1. WI~,KINS,, K. W., HOWANDER, W., ASD CHO- MNIAN, .\. 17.: Chlorothiazid~~ in hyper- te1iaion. Studies on its illode of action. Ann. Sew lTork Ahead. SC. 71: 46.5, 1965. #j. GTHSUN, J. G., .JIL, .jsu Ev-ENS, IV. A., JR. : Clinkal studies on the blood wlu~w. J. C'liu. Invest. 16: 317, 193'7. 6. ~~IHKSON, C. P., em E:RPRT, R. V. : Study of shock iii battle wsualties : Measurcnient of blood vc~lull~e chmgrs iu responses to thern- py. Qllll. surg. 122: 745> 1945. i. FRETS, IX. D., AND SMITIIWICK, 12. H.: Effects of lun~bodorsal spl~lllcilIlicectoll1~ on the blood \~O~UIII~ and LLthiocy;tll:lte space" of patients with essential hypertension. Am. .J. 31. Se. 214: 363, 1947. 8. CHAXDAI,L> 1,. L\., .JK., .\NIl kDEKSON, br. s. : Estilllxtiou of state of hydration by alllnunt of WI tcr available for solution of thioqa- uate. A\~~~. ,J. Tjigest. Ibis. & Kutrititm 1 : 186 .I 934. !). \\:aLSl:R, JL., S&X,DI~, 1). I\:., AN11 (:ROLIXAN. 1 .: \ dii evaluation of radiosulfate for the df~tf~~itiili;rtioll of tile voluule of extracellul:r~ fluid in man and dogs. .J. Clin. Inrcst. 32: 299, 1953. 10. S,(`l1~4LES, o., AND Sf?IIAl,tiS, 8. S.: Silll[llf: :llld :lccur:ltc turthod of dctel~lt~illatioll of clllo~- idc iu biological fluids. tJ. Riol. Cllrnl. 140: Si9,1941. 11. IrAN SLYKE, 1). I).: Stllflics of acidosis; de- trrlrlillntiou of bica rbouatc ronccntmtiou of blood and plnsn~n. J. Biol. Chem. 52: 495, 1922. 12. ~,EARD, S. E., AND FRE:IS, E. 1). : Changes in volume of the ~~:ISIII:I, interstitial illld iu- tt~~lfTllUli3l~ fluid Splf'f's during Ilydratiolr :llld d~ll~d~iltioll ill Iiorlllitl :~i~d cd(matous snh,jfY%s. Am. 6. JIed. 7: 6!)7, 1!)49. 1:1. l,b:\`lw'. Al. P'. : I'liysiolo~i~;tl c~ollsitle~atiotlh nl' ~~~~III;I. ,J. Noullt Sitlai 1 tosp. 19: 571, l!);j`l -. 14. CIWSL~~Y, I\. I'., JK.,, COST~LLO~ C., FREE- Jr&w, ,J. I)., WHIW:~ I). H., .JIz., .\SD Rowe, Ct. C. : Thp neute cfiwts of caarbonic all- hydtxsc inhibitors on sgstenric hrlllody- nnuk. J. Cliu. Iuves;t. 37: SST, 1958. 15. I)I~STAN, H. I?., CIGVMIKGS, Cr. K., C(o1zco11.4~. -1. c., .tND PAGE, 1. 1-1. : h lllc(.ll:lllisl11 of c~lilorothiazitle cnhaucrd efffvtirf~lless ol antihyprrtenaiw gaiiglioplrgic tllwgs. Gil*- culation 19: 360, 1959. 16. TORIAX, L., AFD KEULEAF, P. 11.: Effects 01' hyperteiision on arterial wall elrctrolvtes during desos?eo~ticoatrrollc adlllillistratioll. Circulation Research 4: 6i1, 1956. 17. TAPJA, F. 9., DIiSTAN, 1I. P., S;CIINEC~KI,OTI?, 1~. A., COR~~OKAN, `1. C., ASI) PAGE, T. H.: Ellh:~n~fl effectiveness of ganglionic block- ilig ngyiitb. Latwt 2: 531, 1957. is. ~lER~ruI,r,, .J. P., GLIINAND-HALDO, .\., A\SD C~TORDANO, C. : Etkts of chlorothiazide ou 1~owpincpllrirle rcspollsf~ ill liu~ll2lu Iiypf~v- tension. Clinical Research 6 : 230, IS,%. I!). \VASKO, A., .WD FREIS, E. D. : -1rterial vas~w 1, ~11' respousivruess following chlorothiazidc or nicrcukil diuresis iii iiormotensive sub- jects. Ciwulatiou 10 : 793, 1955. 20. I'ERRT, M. I-I., JR., AND SCHROEDER, H. A. : Studies on control of hypertension. VI. Some evidence of reversal of process during hcxamethoiiiuni and hylralazinr therapy. Circulatioll 8: 528, 1956.