THE EFFECTS OF ANGIOTENSIN-CONVERTING ENZYME-INHIBITORS (ENALAPRIL AND CAPTOPRIL) ON THE RESPONSES OF THE ISOLATED PAPILLARY-MUSCLE OF THE RABBIT TO NIFEDIPINE

In the present experiments the interaction between nifedipine, a dihyd ropyridine calcium channel blocker, and angiotensin-converting enzyme inhibitors (captopril and enalapril) on the isolated electrically stim ulated papillary muscle of the rabbit was studied. Nifedipine (1.3 mum ol.l-1, 3 min) completely abolished the staircase phenomenon. This con centration of nifedipine produced a general, frequency-dependent depre ssion of the response of the papillary muscle by 26-54%. Both enalapri l and captopril (3.37 mumol.l-1, 90 s) by themselves did not qualitati vely alter the staircase phenomenon. This means that in the presence o f enalapril or captopril in the bathing medium this phenomenon was pre sent, but evidently at a lower level, i.e. the response of the muscle to each frequency of the electrical stimulation was always depressed i n comparison with the control response (with no drug in the medium). I n the presence of nifedipine and captopril or enalapril, the staircase phenomenon was completely abolished or preserved, respectively. Cur r esults indicate a possible implication of calcium in the depressive ac tion of both ACE inhibitors on the electrically paced heart muscle of the rabbit. Also, our results suggest that only enalapril possesses pr otective action against nifedipine-induced changes of the staircase ph enomenon.