Myocardial relaxation. V. Postextrasystolic contraction-relaxation in the intact dog heart.

Postextrasystolic (PES) relaxation of the left ventricle was studied in 24 anesthetized dogs using the time constant (tau) of the left ventricular isovolumic pressure decline as an index of global relaxation velocity. Using programmed atrial stimulation and a control RR interval of 500 msec, the "coupling interval/compensatory pause" was varied from 400/600 msec to 250/750 msec, and left ventricular pressure-segment length data from control beats were compared with data from PES beats. Contractile state and fractional shortening increased in the PES beat, but the relaxation time constant remained unchanged (control, 35 +/- 3 msec; PES at 250/750 msec, 36 +/- 3 msec) (p = NS). Pretreatment with propranolol did not qualitatively influence these results. Isoproterenol and calcium were given in doses sufficient to increase the time derivative of isovolumic pressure (maximal positive dP/dt) by an amount equal to that obtained with PES potentiation (approximately 50%); isoproterenol produced a substantial decrease in the relaxation time constant (38 +/- 4 to 30 +/- 6 msec, p less than 0.01), whereas calcium administration produced only a small decrease in the time constant (30 +/- 5 to 27 +/- 5 msec, p less than 0.05). Thus, in the intact dog heart, some positive inotropic interventions augment contraction and speed relaxation, but PES potentiation of contraction is not associated with a change in relaxation velocity.