β-Adrenergic modulation of transient inward current in guinea-pig cardiac myocytes

Transient inward current (I_ti) indicating Ca²⁺-release from the sarcoplasmic reticulum and L-type Ca²⁺-current(I_Ca) were studied in atrial and ventricular myocytes from hearts of adult guinea-pigs by means of whole-cell voltage-clamp. The increase of I_Ca caused by -adrenergic stimulation using isoprenaline (ISO) or related experimental manoeuvres such as superfusion with forskoline (FORSK) was used as a qualitative monitor of an increase of intracellular cAMP. Changes of I_ti were used to manifest changes of sarcoplasmic Ca²⁺-release. In myocytes dialysed with citrate-based (60 mM) pipette filling solution containing 100 M 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 (5·10^–8M) increased the amplitude of spontaneous I_ti and reduced its time-to-peak. The effects of ISO on I_ti developed in parallel to stimulation of I_Ca. In myocytes which did not show spontaneous cyclic Ca²⁺-release in the above condition, this could be evoked de novo by ISO. Spontaneous I_ti was suppressed in the majority of cells by increasing the concentration of EGTA in the dialysing solution to 200 M. Brief (50 ms) activation of I_Ca by voltage steps from –50 to +10 mV usually failed to trigger Ca²⁺-release from the SR. The increase of I_Ca-amplitude upon administration of ISO went ahead with the induction of Ca²⁺-release by brief activation of I_Ca. The effects of ISO could be mimicked by FORSK or intracellular dialysis with 35-cyclic adenosine monophosphate. The effects on I_Ca and SR Ca²⁺-release were dependent on 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 I_ti's. The stimulating effects of ISO or FORSK on Ca²⁺-release were only partially due to an increase of the triggering I_Ca. Ca²⁺-currents too small to trigger Ca²⁺-release before -adrenergic stimulation could evoke Ca²⁺-release after augmentation of intracellular cAMP. Whereas the effects of ISO and FORSK on I_Ca were reversible, the stimulatory effects on Ca²⁺-release persisted after washing out the substances. The results give support to the hypothesis that -adrenoceptor-mediated positive inotropic and arrhythmogenic effects are, at least partly, due to a cyclic AMP-dependent regulatory mechanism modulating sarcoplasmic Ca²⁺-release.This work was supported by the Deutsche Forschungsgemeinschaft (FG Konzell)