[Effect of alpha 1 adrenergic stimulation on myocardial relaxation]

The aim of the present study was to elucidate which was the effect of alpha 1 adrenergic stimulation on myocardial relaxation. Experiments were performed in cat papillary muscles contracting isometrically at 12 beats/min. The effect of phenylephrine (P) in the presence of propranolol (10(-6)M) on myocardial relaxation was compared with that produced by a similar increase in myocardial contractility (+T) elicited by increasing extracellular calcium (Ca2+). An increase of 40% in maximal velocity of contraction (+T) produced by P and Ca2+ elicited in both cases a proportional increase in maximal velocity of relaxation (-T). Therefore the +T/-T ratio remained unchanged. At this contractility level relaxation time (Rt) and time to half relaxation (t1/2) were slight by significantly prolonged by P and not changed by Ca2+. When contractility was increased by approximately 200%, both, P and Ca2+ increased by a less extent -T. Consequently +T/-T augmented from 1.21 +/- 0.04 to 1.73 +/- 0.09 (48.3 +/- 9.5%) with P, and from 1.37 +/- 0.04 to 1.77 +/- 0.15 (31 +/- 12%) with Ca2+. Rt was also significantly increased by both interventions: 21.5 +/- 8.8% (Ca2+) and 24.2 +/- 2.68% (P), whereas t1/2 was significantly increased by P from 215 +/- 11 msec to 282 +/- 12 msec (33 +/- 5.8%) but did not change with Ca2+. Prazosin (10(-6)M) antagonized the effects on contractility and relaxation produced by P. The present results thus indicate that the positive inotropic effect of alpha 1 adrenergic stimulation may be characterized by a prolongation of the early relaxation phase, that was not mimicked by a similar increase in myocardial contractility elicited by increasing extracellular calcium.