Cyclic AMP-independent inhibition of cardiac calcium current by forskolin.

Low-to-moderate concentrations (< or = 3 microM) of forskolin (FSK) stimulated L-type Ca2+ current (ICa,L) and activated Cl- current (IC1) in guinea pig ventricular myocytes investigated under standard whole-cell conditions at 35 degrees. These stimulatory effects reached a steady state after several minutes and smoothly decayed after a short lag period on removal of the drug. Short (2-3 min) exposures to higher concentrations (10-100 microM) of FSK frequently had a multiphasic effect on ICa,L; marked stimulation during the first minute quickly faded during the next 1-2 min, and removal of the drug caused secondary stimulation that lasted for several minutes. Because the amplitude of cAMP-dependent ICl remained stable during the fade and secondary stimulation of ICa,L, the latter modulation of ICa,L seemed to be the result of a cAMP-independent inhibitory action of FSK on Ca2+ channels. Under conditions in which the stimulation of cAMP by FSK was slowed (22 degrees), rapid application of 10-30 microM FSK revealed that inhibition occurred within < 1 sec. In myocytes dialyzed with channel-up-modulating cAMP solution. 0.01-1 microM FSK had no effect on up-modulated currents, whereas high FSK rapidly and reversibly inhibited ICa,L by < or = 42% without affecting ICl. High FSK also inhibited ICa,L in myocytes dialyzed with protein kinase A inhibitor. External but not internal application of the inactive analog 1,9-dideoxy-FSK (30-100 microM) inhibited basal ICa,L. The inhibition was dependent on holding potential and involved a speeding up of ICa,L inactivation and a slowing of recovery from inactivation. We conclude that FSK inhibits cardiac ICa,L by reducing the availability of Ca2+ channels.