Agonist-independent modulation of L-type Ca currents by basal G_s protein activities in single guinea pig ventricular myocytes

The modulation of L-type Ca²⁺ currents (I_{Ca, L}) by the basal activities of G proteins was studied in adult guinea pig ventricular myocytes by whole-cell patch-clamp techniques. With intrapipette guanosine triphosphate (GTP) (100μM), a specific inhibition of G_i proteins by pertussis toxin (PTX) produced an increase in the basal density of I_{Ca, L} (from 11.0 ± 0.8, n = 13, to 25.0 ± 2.0pA/pF, n = 11, at 0mV test potential). In addition, PTX shifted the forskolin (Fsk) concentration-I_{Ca, L} response relation significantly leftward (EC₅₀ = 63.7 ± 12.5 vs 625 ± 75nM). With intrapipette guanosine diphosphate (GDP)βS (1mM), the Fsk-I_{Ca, L} relation was also shifted leftward (EC₅₀ = 197 ± 18.3 vs 781 ± 82.5 nM). However, chronic GDPβS dialysis accelerated the rundown of I_{Ca, L} significantly, suggesting a potential contribution of G_S proteins in maintaining basal I_CaL. In contrast, intra-pipette GTPγS (100μM) produced a transient rise in I_{Ca, L} from 11.0 ± 3.0 to 22.8 ± 7.0 pA/pF (in 3.4 min after whole-cell formation at 0mV, n = 9), presumably through the activation of G_S proteins. It was followed by a gradual decline in I_{Ca, L} (to 15.5 ± 3.5 pA/pF), which was still enhanced by Fsk (EC₅₀ = 1450 ± 98nM), indicating that the current decay was not solely due to rundown but to activation of G_i proteins. G_S, in addition to G_i proteins, show sufficient basal activity to modulate I_{Ca, L} in an agonist-independent manner.