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Endothelin-1 and photoreleased diacylglycerol increase L-type Ca²⁺ current by activation of protein kinase C in rat ventricular myocytes

Jia-Qiang He *, YeQing Pi *, Jeffery W. Walker * and Timothy J. Kamp *¹

1. The amphotericin B-perforated whole-cell patch clamp technique was used to determine the modulation of L-type Ca²⁺ channels by protein kinase C (PKC)-mediated pathways in adult rat ventricular myocytes.

2. Application of 10 nM endothelin-1 (ET-1) increased peak Ca²⁺ current (I_Ca) by 28·2 ± 2·5 % (n = 13) and slowed current decay. These effects were prevented by the endothelin receptor antagonist PD145065 (10 µM) and by the PKC inhibitor chelerythrine (8 µM).

3. To establish if direct activation of PKC mimicked the ET-1 effect, the active and inactive phorbol esters (phorbol-12-myristate-13-acetate and 4-phorbol-12, 13-didecanoate) were tested. Both phorbol esters (100 nM) resulted in a small (10 %) increase in I_Ca, suggesting PKC-independent effects.

4. Bath application of dioctanoylglycerol (diC₈), a diacylglycerol (DAG) analogue which is capable of directly activating PKC, caused a gradual decline in peak I_Ca (50·4 ± 6·2 %, n = 5) and increased the rate of current decay. These effects were unaffected by the PKC inhibitor chelerythrine (8 µM).

5. Intracellular photorelease of caged diC₈ with 3 or 10 s exposure to UV light produced a concentration-dependent increase in peak I_Ca (20·7 ± 8·5 % (n = 8) for 3 s UV and 60·8 ± 11·4 % (n = 13) for 10 s UV), which could be inhibited by chelerythrine.

6. Our results demonstrate that both ET-1 and intracellularly photoreleased diC₈ increase I_Ca by a PKC-mediated pathway, which is in direct contrast to the PKC-independent inhibition of I_Ca produced by bath-applied diC₈. We conclude that specific cellular pools of DAG are crucially important in the regulation of I_Ca by PKC.

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