Effects of manganese chloride, verapamil, and hypoxia on the rate-dependent increase in internal longitudinal resistance of rabbit myocardium.
The effects of high rates of stimulation on the internal longitudinal restivity (Ri) and conduction velocity (theta) were studied on rabbit papillary muscle preparations using a silicon-oil chamber. Increasing the rate from 75 to 150/min caused Ri to rise and theta to decrease. The maximum rate of depolarization and action potential duration were also decreased. At a rate of 300/min the effects were more pronounced. Blockade of the slow inward current (Isi) and of the Na-Ca exchange by MnCl2 (5 mmol/L) did not prevent rate-induced changes in these variable. Verapamil (0.02 mmol/L) was also ineffective. Hypoxia (PO2 = 5.3 kPa) at 75/min induced changes in Ri and theta which were similar to those recorded at 150/min under aerobic conditions. The effects of high rates of stimulation were potentiated under hypoxia. From the present results it is suggested that Isi and the Na-Ca exchange are not the main determinants of the rate-induced increase in Ri, which could be determined by other intracellular Ca-release mechanisms or by a decrease in myoplasmic pH.
Laboratorio de Electrofisiología, Instituto de Cardiología y Cirurgía Cardiovascular, Vedado, Habana, Cuba.
J
J
Alvarez
G
G
Rousseau
F
F
Dorticós
J
J
Morlans
D000818:Animals; D002712:Chlorides; D004553:Electric Conductivity; D004558:Electric Stimulation; D000860:Hypoxia / Q000503:physiopathology*; D066298:In Vitro Techniques; D008345:Manganese / Q000494:pharmacology*; D017895:Manganese Compounds; D009200:Myocardial Contraction / Q000187:drug effects; D010210:Papillary Muscles / Q000187:drug effects / Q000502:physiology* / Q000503:physiopathology; D011817:Rabbits; D016276:Ventricular Function; D014700:Verapamil / Q000494:pharmacology*
D002712:Chlorides; D017895:Manganese Compounds; D008345:Manganese; D014700:Verapamil; C025340:manganese chloride