Opposite effects of angiotensin II and the protein kinase C activator OAG on cardiac Na⁺ channels

Summary

Elementary Na⁺ currents were recorded at 19°C in cell-attached and inside-out patch-clamp experiments to study the influence of the vasoactive peptide angiotensin II (A II) and of the diacylglycerol analogue OAG (1-oleoyl-2-acetyl-snglycerol) on open probability and gating properties of single cardiac Na⁺ channels from cultured neonatal rat cardiocytes. Treating the cardiocytes with A II caused Na⁺ channel activation: reconstructed peak I_Na increased to 137 ± 17.5% of control at 3 μmol/liters and to 176 ± 42% at 30 μmol/liter. This NP_o increase developed without major changes in open state and burst activity, even at 30 μmol/liter. OAG (6 μmol/liter) did not mimic this A II action. By contrast, OAG treatment of the cardiocytes had the opposite effect on NP_o and diminished reconstructed peak I_Na to 67 ± 4.9% of the control. The putative protein kinase C inhibitor staurosporine (0.2 μmol/liter) abolished this I_Na depression and led to a normalization of NP_o. OAG had the same effect on isolated Na⁺ channels. Exposure of the cytoplasmic surface of inside-out patches to 1 μmol/liter OAG reversibly depressed, in the simultaneous presence of 50 μmol/liter Mg-ATP, the reconstructed peak I_Na to 40 ± 9.7% of the control but left i _unit, τ _open and burst activity unaffected. No NP_o depression was obtained in the absence of Mg-ATP indicating that Mg-ATP may serve as phosphate donor. Obviously, after phosphorylation by protein kinase C, cardiac Na⁺ channels attain a reduced open probability but appear to preserve their kinetic properties. It is also concluded that activation of protein kinase C is not the mechanism underlying the A II induced channel activation.