The effects of trimetoquinol on the intact rabbit heart and myocardial adenylate cyclase activity: evidence for spare myocardial beta receptors.

We have compared and contrasted the actions of (-)isoproterenol and (+/-) trimetoquinol on rabbit heart preparations. In the presence of either GTP or Gpp[NH]p (guanosine-5'-(beta, gamma imino) triphosphate), trimetoquinol displayed partial agonist activity in stimulating adenylate cyclase activity in a particulate rabbit heart preparation. Trimetoquinol enhanced adenylate cyclase activity 20% or 65% of the maximum obtainable by isoproterenol in the presence of GTP or Gpp[NH]p respectively. In the presence of GTP, concentrations of catecholamines required to enhance cyclase activity 15% of the maximum obtainable with isoproterenol (EC15) were 2.0 X 10(-7) M and 5.5 X 10(-8) M for trimetoquinol and isoproterenol, respectively. In the presence of Gpp[NH]p EC30 values were 2.0 X 10(-7) and 3.5 X 10(-8) M for trimetoquinol and isoproterenol respectively. Trimetoquinol also displayed partial agonist activity for the ability to increase cAMP levels in the isolated perfused rabbit heart. By contrast trimetoquinol was equieffective to isoproterenol at increasing tension development and rate of contraction of the isolated perfused heart. Concentrations of catecholamines required to increase tension and rate of contraction 50% of the maximum obtainable with isoproterenol were 1.5 X 10(-7) M and 1.7 X 10(-8) M for trimetoquinol and isoproterenol, respectively. These data show that only a partial stimulation of adenylate cyclase activity and cAMP levels by trimetoquinol is sufficient to produce maximal changes in mechanical activity of the heart.(ABSTRACT TRUNCATED AT 250 WORDS)