ARGININE VASOPRESSIN-INDUCED POTENTIATION OF UNITARY L-TYPE CA2+ CHANNEL CURRENT IN GUINEA-PIG VENTRICULAR MYOCYTES

The effects of arginine vasopressin (AVP) on L-type Ca2+ channels were studied by recording single-channel activity from cell-attached patches on isolated guinea pig ventricular myocytes, with 100 mmol/L Ba2+ used as the charge carrier. Bath application of AVP (100 nmol/L) reversibly increased channel open probability by a factor of 2.92+/-1.43 (n=15) because of the increased number of channel openings and increased open times. AVP did not change the amplitudes of single-channel currents (1.17+/-0.10 pA in the control condition and 1.12+/-0.11 pA after AVP, at +20 mV; n=6). In our experimental conditions, in which myocytes were bathed in Ca2+-free high-potassium solutions, AVP-induced potentiation was observed without changes in [Ca2+](i) measured by fura 2 fluorescence signals (estimated [Ca2+](i), approximate to 80 nmol/L). The AVP-induced increase in channel open probability was abolished by OPC-21268 (8 mu mol/L), a specific blocker of V-1 receptor, but not by a V-2 blocker, OPC-31260 (5 mu mol/L). AVP-induced potentiation was also suppressed by a broad-spectrum protein kinase inhibitor, H7 (100 mu mol/L, bath application), but not by H89 (1 mu mol/L), a blocker with high specificity to protein kinase A. AVP application after the treatment by phorbol eater (phorbol 12-myristate 13-acetate, 100 nmol/L for 1 hour) failed to potentiate the channel activity. These results raised the possibility that protein kinase C might be involved during signal transduction. Our results provide direct evidence that AVP potentiates cardiac L-type Ca2+ currents via V-1 receptor stimulation.