Mechanisms for the Positive Inotropic Effect of α1-Adrenoceptor Stimulation in Rat Cardiac Myocytes

α1-Adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are an increase in myofilament Ca sensitivity and action potential prolongation, which can increase net Ca entry into cells. In adult rat ventricular myocytes (bath Ca, 1 mM; stimulated at 0.2–0.5 Hz), the drug 4-aminopyridine and the whole-cell voltage clamp have been used to control Ca entry and differentiate between the two mechanisms. At 22–23°C the specific α1-adrenoceptor agonist methoxamine (100 μM) prolonged action potential duration at 50% repolarization from 55±2 to 81±5 msec, delayed time to peak contraction, and increased shortening amplitude from 5.3±0.6 to 7.8±1 μm (n=18). Reduction of the transient outward current and other K currents by methoxamine was the major cause of action potential prolongation in rat myocytes with little change in the L-type calcium current. Block of the transient outward current with 2 mM 4-aminopyridine prolonged action potential duration from 52±6 to 98±12 msec and increased unloaded cell shortening from 2.9±0.4 to 6.6±0.6 p.m (n=4). Subsequently, methoxamine no longer increased cell shortening, although significant potentiation of twitch amplitude was still seen after a brief rest interval. In voltage-clamp experiments, with 70–500-msec pulses, although membrane currents were reduced, methoxamine had no positive inotropic effect and reduced cell shortening from 5.3±0.7 to 4.97±0.8 μm at pulse potentials positive to −40 mV. Similar al-adrenoceptor responses were observed at 35°C during action potential and voltage-clamp experiments, which could be blocked by 10 μM prazosin. In myocytes loaded with the Ca indicator indo-1, α1-adrenoceptor stimulation or 4-aminopyridine both increased cell contraction and intracellular Ca transients by similar amounts. As in unloaded cells, prior exposure to 4-aminopyridine prevented any inotropic effect of methoxamine without changing the systolic intracellular Ca transient. The results indicated that under our experimental conditions positive inotropy in rat cardiomyocytes on exposure to α1-adrenoceptor agonists was strongly correlated with the action potential prolongation that accompanied K current reduction. In addition, modulation of K channels could occur independent of changes in contractility and/or [Ca];.