Metabolic protection by verapamil during graded coronary flow reduction independent of effect on baseline systolic function. Separation of mechanical and ionic markers of ischemia.

Pretreatment with the calcium channel-blocking agent verapamil lowers the coronary flow associated with the first rise in myocardial extracellular potassium [( K+]e). The mechanisms underlying this effect are unclear. It is not known whether this effect is a manifestation of verapamil-induced reduction in baseline cardiac work before the reduction in coronary flow, is dependent on a selective depression of contractility within the low-flow region, or is independent of an effect on myocardial work. This study was performed to determine the relations between changes in regional contractility and [K+]e before and after verapamil (0.2 mg/kg followed by 6.5 micrograms/kg/min) when left anterior descending (LAD) coronary flow is progressively reduced and when verapamil-induced alterations in baseline myocardial work are prevented by atrial pacing and by dobutamine (4.3 +/- 2.2 micrograms/kg/min) to maintain systemic arterial blood pressure and contractility. Before verapamil-dobutamine, myocardial [K+]e rose and regional contractility fell when LAD coronary flow was reduced to 87.7 +/- 9.6% and 83.4 +/- 7.4%, respectively, of the unrestricted control value (p = NS). After verapamil-dobutamine, the threshold flow for rise in [K+]e decreased to 56.4 +/- 13.5% of the unrestricted control flow (p = 0.003), but the threshold flow for regional contractility fall was unchanged (84.8 +/- 11.3%). Our results indicate that the protective effect of verapamil on preventing ischemia-induced [K+]e release is not dependent on a reduction in baseline myocardial work. In this setting, calcium channel blockade by verapamil results in a dissociation between the ionic and mechanical events that occur when coronary flow is reduced.