α-Adrenergic stimulation of trans-sarcolemma electron efflux in perfused rat heart. Possible regulation of Ca2+-channels by a sarcolemma redox system

The role of trans-sarcolemma membrane electron efflux in the α-adrenergic control of Ca²⁺ influx in perfused rat heart was examined. Electron efflux was measured by monitoring the rate of reduction of extracellular ferricyanide and compared with changes in contractility, as an indirect assessment of changes in cytoplasmic Ca²⁺ concentration. Methoxamine and phenylephrine each increased the rate of ferricyanide reduction from 80 to approx. 114 nmol/min per g wet wt. of heart, with half-maximal activation occurring at 10 μM for each agonist. Activation of the rate of ferricyanide reduction by both 10 μM methoxamine and 10 μM phenylephrine was blocked by the α-adrenergic antagonist, phenoxybenzamine, but not by the β-antagonist, propranolol. Stimulation of the rate of ferricyanide reduction by the α-agonist coincided with the increase in contractility, each reaching maximum values at approx. 80 s. Removal of the α-agonists led to parallel decreases in contractility and the rate of reduction, each returning to pre-stimulation values in approx. 400 s. In addition, the relationship between Ca²⁺ and ferricyanide reduction was examined. Perfusion of the heart with medium containing 6 mM CaCl₂ significantly increased contractility and the rate of ferricyanide reduction. Perfusion of the heart with low Ca²⁺ diminished contractility, did not affect the rate of ferricyanide reduction, but amplified the stimulatory effect of methoxamine on this rate. The increase in ferricyanide reduction by α-adrenergic agonists resulted from a change in the apparent V_max, indicative of an increase in electron efflux sites in the plasma membrane. It is concluded that α-adrenergic control of electron efflux closely parallels changes in contractility and therefore changes in the cytoplasmic concentration of Ca²⁺. The data suggest that α-agonist-mediated changes in electron efflux may lead to Ca²⁺ influx.