title: Decrease in time to peak tension produced by calcium blockers and isoproterenol. Its dependence on extracellular calcium.
abstract: The present experiments were designed to analyze the effect of verapamil, nifedipine and isoproterenol on myocardial contractility (+dT/dtmax) and time to peak tension (TTP) and its dependence on extracellular calcium concentration (Ca2+). The experiments were performed on cat papillary muscles from reserpinized cats. At (Ca2+) 1.34 mM the negative inotropic effect of cumulative doses of verapamil and nifedipine and the positive inotropic effect of isoproterenol occurred associated with a significant and dose-dependent decrease in TTP. The decrease in TTP produced by calcium blockers was completely reversed by the addition of calcium in a close parallelism with the recovery in myocardial contractility: At (Ca2+) 1.34 mM; verapamil 10(-6)M significantly decreased TTP from 299 +/- 14 msec to 250 +/- 11 msec. At (Ca2+) 15 mM values for TTP were 285 +/- 7 msec (control) and 291 +/- 5 msec (verapamil). Nifedipine 5 X 10(-7) M decreased TTP from 343 +/- 20 msec to 246 +/- 4 msec (Ca 1.34 mM). Values of TTP at calcium 15 mM were 296 +/- 6 msec (control) and 300 +/- 17 msec (nifedipine). On the contrary, the significant decrease in TTP produced by isoproterenol 10(-5)M at (Ca2+) 1.34 from 266 +/- 14 msec to 222 +/- 14 msec was still present at (Ca2+) 15 mM: 233 +/- 8 msec (control) and 220 +/- 8 msec (isoproterenol). The results suggest that the decrease in TTP produced by calcium blockers and isoproterenol is mediated by different mechanisms.
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002415:Cats; D004305:Dose-Response Relationship, Drug; D007545:Isoproterenol / Q000494:pharmacology*; D009200:Myocardial Contraction / Q000187:drug effects*; D009543:Nifedipine / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology*
chemical_list: D014700:Verapamil; D009543:Nifedipine; D007545:Isoproterenol; D002118:Calcium