Abstract
We determined the duration of the effect of a 100 mg/kg dose of isoproterenol on total and isoenzyme lactic dehydrogenase (LDH) activities in the rat heart. Maximum reduction of total heart LDH activity occurred between the first and third day following isoproterenol. Heart LDH isoenzymes 3, 4, and 5 increased to a maxium on the second day and gradually returned to the control level by the tenth day. The LDH 1 and 2 isoenzyme activities were reduced over this same period. Changes in total heart LDH and in the ratios of LDH-H and LDH-M subunits were used as measures of the potential blocking effects of pronethalol and phentolamine on the activity of isoproterenol, epinephrine, norepinephrine, and phenylephrine. Results were essentially the same whether expressed as total LDH activity or as H/M ratios. Phentolamine blocked the effects of epinephrine, norepinephrine, and phenylephrine, whereas pronethalol was ineffective with all the sympathomimetics. The effects of phentolamine and pronethalol were also determined by measuring gross myocardial lesions produced by norepinephrine or isoproterenol. With the exception of a trend toward protection by pronethalol against the effects of larger doses of isoproterenol, the results obtained by using lesion grades were essentially the same as those obtained with total LDH or LDH isoenzyme measurements.
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