Differences between the effects of phenylephrine and other inotropic interventions on post-rest contraction in guinea-pig ventricular strips.

The influence of several inotropic interventions on the strength of the post-rest contraction (PCR) was studied in isolated guinea-pig ventricular strips. In preparations stimulated at 1 Hz and incubated in 3.6 mM Ca++, the PRC-maximum potentiation was obtained after a rest interval of 2 sec (optimum resting time: ORT); a post-rest potentiation was present until 15 sec of rest. An increase in stimulation rate and in Ca++ concentration induced a shift in the ORT towards 15-20 sec, and an increase in the potentiation of PRC. At a low stimulation rate (0.33 Hz) the PRC was lower than steady-state contraction (SSC) at all rest intervals tested. Phenylephrine (5 X 10(-6) M) did not modify the ORT either at 1 or 2.5 Hz, and reduced the height of PRC at 0.33, 1 and 2.5 Hz. These results are discussed in terms of the different effects induced by the various inotropic interventions on cellular calcium availability.