Functional evidence for the regulation of cytosolic Ca2+ activity via V-1A-receptors and beta-adrenoceptors in rat CCD

In freshly isolated rat CCD segments, the effects of arginine vasopressin (AVP), oxytocin (OT), adrenaline (Ad), and their specific receptor agonists and antagonists on the intracellular Ca2+ activity ([Ca2+](i)) were measured using the Ca2+ sensitive dye Fura-2 as fluorescence indicator. We observed that AVP, the V-1-receptor agonist [Phe(2)Om(8)] vasotocin ([Phe(2)]OVT), and OT increased [Ca2+](i) biphasically. AVP (n = 9) and OT (n = 8) induced increases in [Ca2+](i) were completely blocked by the V-1A-receptor antagonist d(CH2)(5)Tyr(Me)(2)AVP. However, neither the V-2-receptor agonist [Val(4)-D-Arg(8)]AVP (100 nM, n = 5), nor the OT-receptor agonist [Thr(4),Gly(7)]OT (100 nM, n = 5) nor forskolin (1 mu M, n = 4 and 10 mu M, n = 5) did significantly change [Ca2+](i). Ad and the beta-adrenoceptor agonist isoproterenol (ISO) increased [Ca2+](i), which was not mimicked by the alpha(2)-adrenoceptor agonist clonidine (1 mu M, n = 10) or the alpha(1)-adrenoceptor agonist phenylephrine (1 mu M, n = 5). The beta-adrenoceptor antagonist propranolol (1 mu M) completely blocked this Ad (1 mu M, n = 4) induced [Ca2+](i) increase. Insulin (INS 10 nM, n = 8), endothelin (ET 1 mu M, n = 6), and angiotensin II (Ang II 1 pM to 10 nM; each n = 4) had no significant effect on [Ca2+](i). Considering the present results we propose a V-1A-receptor and beta-adrenoceptor dependent modulation of [Ca2+](i) in rat CCD.