Direct block of calcium channels by dioctanoylglycerol in pregnant rat myometrial cells.

The effect of 1,2-dioctanoyl-sn-glycerol (DiC8), a diacylglycerol and potent protein kinase C (PKC) activator, on the voltage-dependent slow (L-type) Ca2+ current [lCa(L)] was examined using whole-cell voltage clamp of myometrial cells freshly isolated from late pregnant rats. Bath application of DiC8 (25 microM) decreased ICa(L) by 50.7 +/- 2.4% (n = 22). The effect was reversible and dose dependent (IC50 of 15.3 microM). The effect of DiC8 was not reversed or prevented by application of calphostin C, a selective PKC inhibitor. In addition, 1-oleoyl-2-acetyl-sn-glycerol, another PKC activator, did not produce an inhibitory effect on ICa(L), even at a concentration of 100 microM; ICa(L) was actually slightly stimulated (10.6 +/- 5.1%, n = 6). The steady state inactivation curve for ICa(L) was shifted to the left by DiC8 application (by approximately 15 mV at 25 microM), whereas the activation curve was not affected; the shift should produce a voltage-dependent block. DiC8 decreased ICa(L) progressively during repetitive step depolarizations (use-dependent block). We conclude from these results that (a) DiC8 inhibits ICa(L) independently of PKC in uterine muscle cells and (b) DiC8 preferentially acts on the inactivated state and/or open state of the L-type Ca2+ channels. Therefore, caution must be exercised when studying PKC actions on ion channel regulation using DiC8 as a PKC activator.