Comparison of slow response action potentials from normal and hypertrophied myocardium

Previous studies have shown that papillary muscles from hypertrophied cat right ventricles (RVH) exhibit altered mechanical properties which may be associated with defects in the excitation-contraction coupling process. Since calcium influx [as slow inward current (Isi) during the cardiac action potential is thought to be a major determinant of contractile state, we compared Isi-mediated slow response action potentials ( SRAPs ) in papillary muscles from cats with RVH, induced by chronic pulmonary artery constriction, to SRAPs from sham-operated controls. The results show that 1) when depolarized by elevated extracellular potassium (K+o, 22 mM), RVH muscles became inexcitable (as defined here) significantly faster than control muscles; 2) inexcitable RVH muscles required significantly more isoproterenol than controls to restore slow response activity; 3) at all isoproterenol concentrations tested, SRAPs from RVH muscles were reduced in amplitude and duration compared with controls; 4) SRAPs evoked by long duration stimulus pulses in the absence of isoproterenol were also markedly reduced in RVH; and 5) the relationship between resting potential and K+o was the same in both groups. If the alterations in SRAPs observed in RVH are produced by a smaller Isi, this change may be associated with the diminished inotropic state of cardiac muscle caused to hypertrophy due to pressure overload.