MECHANISMS FOR THE POSITIVE INOTROPIC EFFECT OF ALPHA-1-ADRENOCEPTOR STIMULATION IN RAT CARDIAC MYOCYTES

alpha-1-Adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are an increase in myofilament Ca2+ sensitivity and action potential prolongation, which can increase net Ca2+ entry into cells. In adult rat ventricular myocytes (bath Ca2+, 1 MM; stimulated at 0.2-0.5 Hz), the drug 4-aminopyridine and the whole-cell voltage clamp have been used to control Ca2+ entry and differentiate between the two mechanisms. At 22-23-degrees-C the specific alpha-1-adrenoceptor agonist methoxamine (100-mu-M) 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-mu-m (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-mu-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-mu-m at pulse potentials positive to -40 mV. Similar alpha-1-adrenoceptor responses were observed at 35-degrees-C during action potential and voltage-clamp experiments, which could be blocked by 10-mu-M prazosin. In myocytes loaded with the Ca2+ indicator indo-1, alpha-1-adrenoceptor stimulation or 4-aminopyridine both increased cell contraction and intracellular Ca2+ 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 alpha-1-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 [Ca2+]i.