Effects of dextrose, verapamil and magnesium during hypoxia in myocardial tissue

We studied the effects of superfusion of canine heart muscle tissue with a solution that mimicks hypoxia, acidosis and hyperkalemia (altered Tyrode's solution). Contracture (rise in resting tension) develops much sooner (5.2 +/- 0.8 vs. 30-40 min in 5 mM dextrose) in the absence of dextrose. High dextrose (55 mM) stabilizes the rise in tonic tension and protects against the action potential shortening during such superfusion. Presence of verapamil (1-1.5 microM) during altered Tyrode's superfusion considerably lessens the magnitude of the increase in tonic tension (31.7 +/- 8.6 vs. 129.5 +/- 32.6 mg in the control). Presence of high magnesium (5 mM) during altered Tyrode's superfusion also offers some protection against tonic tension increase (12.6 +/- 3.6 mg rise in tonic tension vs. 129.5 +/- 3.2 mg in the control), action potential shortening, and amplitude decrease. These results suggest that (a) magnesium and verapamil both have significant effects on the cellular calcium uptake, and (b) anaerobic metabolism utilizing either glycogen or exogenous glucose is capable of preventing contracture during ischemia.