THE EFFECT OF HORMONES AND PEPTIDES INVOLVED IN WATER-BALANCE ON RAT COLONIC MOTILITY IN-VITRO

We have previously shown that restriction of water intake decreased st ool frequency and stool weight in volunteers. The aim of this study wa s to investigate whether these effects of thirst could be mediated by an action of systematically released hormones on colonic smooth muscle . Using isolated colonic smooth muscle strips the effect of arginine-v asopressin (AVP), angiotensin II (ANG II) and aldosterone on rat colon ic motility in vitro was investigated. AVP (10(-12)-10(-10) mol/l) and aldosterone (3 x 10(-10)-3 x 10(-8) mol/l) and physiological hormonal concentrations of ANG II (10(-13)-10(-10) mol/l) had no effect on eit her basal activity, direct stimulation of colonic smooth muscle or neu rally stimulated contractions using carbachol 10(-7)-3 x 10(-5) mol/l or neurally stimulated contractions using electrical field stimulation at various stimulation frequencies (1-10 pps, 1 ms, 40 V). ANG II in higher concentrations (10(-7)-10(-6) mol/l) increased basal activity a nd neurally mediated contractions. Accordingly, ANG II (10(-6) mol/l) caused a prestimulation but did not increase the maximum contractile e ffect of carbachol. The response to ANG II was not affected by atropin e (10(-6) mol/l). TTX (10(-6) mol/l) and N-nitro-L8-arginine (L-NNA) ( 3 x 10(-4) mol/l) stimulated basal muscular activity but did not affec t the maximum contractile effect of ANG II. Systemic serum concentrati ons of AVP, aldosterone and ANG II are presumably not involved in thir st-induced colonic motility changes. The ANG II effect in higher conce ntrations is mediated by a direct stimulatory smooth muscle effect and /or by facilitating neuronal liberation of acetylcholine. These higher concentrations of ANG II could be reached when ANG II is acting as a neurotransmitter or through a local renin-angiotension system.