Endothelin activates voltage‐dependent Ca2+ current by a G protein‐dependent mechanism in rabbit cardiac myocytes.

1. Endothelin is a vasoactive peptide released from vascular endothelial cells which has potent cardiac inotropic effects. We examined the effect of endothelin on the verapamil‐sensitive Ca2+ current (ICa) in enzymatically dispersed rabbit ventricular myocytes. 2. Using the whole‐cell voltage clamp technique with a standard dialysing pipette solution, the application of extracellular endothelin (20 nM) did not increase the peak ICa, but in fact caused a small reversible decline (903 +/‐ 109 pA without endothelin, 727 +/‐ 95 pA with endothelin (means +/‐ S.E.M., n = 14, P less than 0.05)). 3. If GTP (100 microM) was added to the pipette solution, the extracellular application of endothelin (0.2 or 20 nM) caused a large, reproducible increase in peak ICa (871 +/‐ 85 pA without endothelin, 1230 +/‐ 110 pA with 20 nM‐endothelin (n = 10, P less than 0.05). The endothelin enhancement of ICa occurred after a delay of approximately 3‐4 min at room temperature. 4. The GTP requirement for the endothelin effect on ICa suggests that its effect may be mediated through a G protein‐dependent pathway. To investigate this further, experiments were performed with pipette solutions containing guanosine‐5'‐O‐(2‐thiodiphosphate) (GDP beta S), a GDP analogue which inhibits G protein cycling. With the addition of GDP beta S (0.5‐5.0 mM) to the pipette solution (along with 100 microM‐GTP), the effect of endothelin on peak ICa was blocked (1062 +/‐ 86 pA without endothelin, 1170 +/‐ 134 pA with endothelin (n = 11, P greater than 0.05)). 5. Incubation of myocytes with pertussis toxin (500 ng/ml) prevented the partial ACh‐induced reversal of the isoprenolol enhancement of ICa. However, this identical treatment failed to block the endothelin enhancement of the voltage‐dependent Ca2+ current (n = 4). 6. Taken together, these results confirm that while the effect of endothelin in rabbit cardiac ventricular myocytes is mediated through a G protein‐dependent pathway, the G protein involved is pertussis toxin‐insensitive.