CAMP-DEPENDENT EFFECTS OF VASOPRESSIN AND CALCITONIN ON CYTOSOLIC CALCIUM IN RAT CCD

Fura 2 fluorescence measurements were carried out on microperfused rat cortical collecting ducts (CCD) to investigate the effect of adenosin e 3',5'-cyclic monophosphate (cAMP) and adenylate cyclase-stimulating hormones on free cytosolic calcium ([Ca2+](i)). Forskolin, 3-isobutyl- 1-methylxanthine, and 8-(4-chlorophenylthio)-cAMP (CPT-cAMP) all trigg ered marked and sustained [Ca2+](i) variations. Maximal increases elic ited by 100 mu M CPT-cAMP amounted to 101 +/- 11 nM (mean +/- SE, n = 18). This effect was mostly dependent on the presence of basolateral c alcium and totally independent of luminal calcium. It remained unchang ed in CCD perfused with sodium-free luminal fluid (82 +/- 10 nM, n = 5 ), pretreated with 1 mM bath ouabain (113 +/- 20, n = 4), or superfuse d with sodium-free hath in the presence of ouabain (82 +/- 22, n = 5). The V-2 agonist 1-desamino-8-D-arginine vasopressin (DDAVP, 10 nM) in creased [Ca2+](i) by 57 +/- 5 nM (n = 27), a value 40% lower than that achieved with 10 nM AVP (141 +/- 7, n = 34) but similar to that obser ved with AVP + a V-1a antagonist (57 +/- 6, n = 6). Significant effect s could also be obtained with 200 pM DDAVP (31 +/- 6, n = 8) and argin ine vasopressin (AVP) (72 +/- 6, n = 16). Rat calcitonin also raised [ Ca2+](i) by 43 +/- 10 (n = 8) and 66 +/- 8 nM (n = 17) at 1 and 10 nM, respectively, and its effect was not additive to that of CPT-cAMP. Ca lcitonin and DDAVP effects, like those of CPT-cAMP and forskolin, were nearly abolished in Ca2+-free bath, but AVP action on intracellular r elease persisted. These results show that, in rat CCD, cAMP effects on [Ca2+](i) mainly result from basolateral calcium entry. In contrast t o rabbit CCD the mechanism is independent on Na reabsorption and basol ateral Na+/Ca2+ exchange. Calcitonin and DDAVP effects on [Ca2+](i) ar e probably secondary to increased cAMP production.