Radioligand binding studies of the cardiac arginine vasopressin (AVP) receptor, together with studies on the AVP-evoked alterations in the [Ca2+]i levels, were undertaken using primary cultures of neonatal rat cardiomyocytes. Rapid, reversible, specific, high-affinity and low-capacity binding sites were detected for the agonist, [3H]AVP, and the V1 selective antagonist, d(CH2)5 Tyr (Me)-[3H]AVP (V1 antagonist), radioligands. The V2 selective antagonist radioligand, d(CH2)5 D-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 degrees 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 [Ca2+]i in a concentration-dependent manner as judged by fura-2 fluorescence. This was completely attenuated by inclusion of the V1 antagonist. The maximal increase in [Ca2+]i 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).(ABSTRACT TRUNCATED AT 250 WORDS)