Vasopressin Increases Cytosolic Free [Ca2+] in the Neonatal Rat Cardiomyocyte

Radioligand binding studies of the cardiac arginine vasopressin (AVP) receptor, together with studies on the AVP-evoked alterations in the [Ca2+1i levels, were undertaken using primary cultures of neonatal rat cardiomyocytes. Rapid, reversible, specific, high-affinity and lowcapacity binding sites were detected for the agonist, [3H]AVP, and the V1 selective antagonist, d(CH2)5 Tyr (Me)-[3H]AVP (V, antagonist), radioligands. The V2 selective antagonist radioligand, d(CH2)5 D-Ile Ile des-Gly NH2-[3H]AVP, showed very little binding even at very high concentrations. [3H]AVP and [3H]V1 antagonist specific binding attained equilibrium in 10 minutes at 37°C. The Kd and Bmax values (mean±SEM) were [3H]AVP: Kd 1.44±0.18 nM; Bmax 5,253±590 sites/cell; [3H]V1 antagonist: Kd 0.96±0.10 nM; Bmax 6,869±485 sites/cell. KI values for a series of AVP-related peptide analogues and antagonists determined by competitive inhibition of [3H]AVP binding were consistent with the saturation data. The results suggest that these cells possess a homogeneous population of V1 subtype AVP receptors. AVP increased [Ca21]1 in a concentration-dependent manner as judged by fura-2 fluorescence. This was completely attenuated by inclusion of the V, antagonist. The maximal increase in [Ca21]1 evoked by AVP from a resting level of 60±5 nM was less (250±35 nM) in comparison to the maximal response evoked by angiotensin II (2,337±640 nM). However, the EC50 value for AVP (0.8±0.2 nM)-evoked increase in [Ca2"]i was significantly lower than that for angiotensin II (2.3±0.9 nM). Thus, the identification of V1 receptors coupled to increases in myocyte [Ca2+]j appears to provide, at the cellular level, a direct role for AVP in the regulation of contractility of neonatal rat cardiomyocytes. (Circulation Research 1991;69:239-245)