Beta-adrenergic modulation of transient inward current in guinea-pig cardiac myocytes. Evidence for regulation of Ca2(+)-release from sarcoplasmic reticulum by a cyclic AMP dependent mechanism

Abstract

Transient inward current (Iti) indicating Ca2(+)-release from the sarcoplasmic reticulum and L-type Ca2(+)-current (ICa) were studied in atrial and ventricular myocytes from hearts of adult guinea-pigs by means of whole-cell voltage-clamp. The increase of ICa caused by beta-adrenergic stimulation using isoprenaline (ISO) or related experimental manoeuvres such as superfusion with forskolin (FORSK) was used as a qualitative monitor of an increase of intracellular cAMP. Changes of Iti were used to manifest changes of sarcoplasmic Ca2(+)-release. In myocytes dialysed with citrate-based (60 mM) pipette filling solution containing 100 microM EGTA spontaneous transient inward currents were recorded at a constant holding potential of -50 mV in the majority of myocytes. Superfusion with a solution containing ISO (greater than or equal to 5 x 10(-8) M) increased the amplitude of spontaneous Iti and reduced its time-to-peak. The effects of ISO on Iti developed in parallel to stimulation of ICa. In myocytes which did not show spontaneous cyclic Ca2(+)-release in the above condition, this could be evoked de novo by ISO. Spontaneous Iti was suppressed in the majority of cells by increasing the concentration of EGTA in the dialysing solution to 200 microM. Brief (50 ms) activation of ICa by voltage steps from -50 to +10 mV usually failed to trigger Ca2(+)-release from the SR. The increase of ICa-amplitude upon administration of ISO went ahead with the induction of Ca2(+)-release by brief activation of ICa. The effects of ISO could be mimicked by FORSK or intracellular dialysis with 3'5'-cyclic adenosine monophosphate. The effects on ICa and SR Ca2(+)-release were dependent o the concentration of the stimulating substance. In a given cell changing superfusion from a low to a high concentration of ISO or FORSK resulted in an increase of the number of Ca2(+)-release events per number of Ca2(+)-currents elicited and a shortening of time-to-peak of Iti's. The stimulating effects of ISO or FORSK on Ca2(+)-release were only partially due to an increase of the triggering ICa. Ca2(+)-currents too small to trigger Ca2(+)-release before beta-adrenergic stimulation could evoke Ca2(+)-release after augmentation of intracellular cAMP. Whereas the effects of ISO and FORSK on ICa were reversible, the stimulatory effects on Ca2(+)-release persisted after washing out the substances. The results give support to the hypothesis that beta-adrenoceptor-mediated positive inotropic and arrhythmogenic effects are, at least partly, due to a cyclic AMP-dependent regulatory mechanism modulating sarcoplasmic Ca2(+)-release.