Effect of Temperature on Excitation-Contraction Coupling of the Bullfrog Atrium

Abstract

Numerous studies have been reported on the effects of temperature on the electrical and mechanical properties of myocardium, and the augmentation of tension in low temperature has been generally considered to be simply due to a marked prolongation of the membrane action potential. In recent years, however, several kinds of the membrane currents such as I_Na1, I_Na2, I_Ca, I_K1, I_K2, I_X1, I_X2, and I_C1 were discovered on the myocardium and the complicated processes of membrane excitation-contraction coupling (E-C coupling) were greatly analized in relation with the roles of these membrane currents and sarcoplasmic reticulum. Accordingly, the mechanisms of the inotropic effect of low temperature must also be re-examined on the basis of these lately developed knowledges. The present study, therefore, undertaken to elucidate fundamental mechanisms of the action of temperature upon the membrane currents and E-C coupling processes, by examining the frog heart muscle with voltage clamp technique. The preparation used in the present experiments was the atrial muscle bundle isolated from the bullfrog (Rana cathesbiana). The thin muscle bundle was about 7mm in length and 0.6-0.8mm in diameter. The membrane potential, current and contractile tension were measured by means of the double glycerol-gap technique in voltage clamped conditions. The muscle chamber consisted of five compartments (1, 2, 3, 4 and 5). The large terminal compartments (1 and 5) were flowed with 200mM KCI solution, the intermediate compartments (2 and 4) with isotonic glycerol solution, and the central compartment with Ringer or test solutions. The width of glycerol gap was 1mm, and that of the central compartment ranged from 0.3 to 0.6mm. The membrane potential was obtained as the potential difference between the central and terminal compartments (3 and 5), and the cur-rent, from the central compartment (3) which was kept at nearly earth potential by the aid of an operational amplifier (Philbrick, P65AU). The muscle tension was measured by the aid of a strain gauge (Nihon Kohden, SB-1T), the sensitive arm of which was connected to the end of muscle bundle in the terminal compartment (5). These potential, current and tension were recorded simultaneously utilizing by a triad-beam oscilloscope (Nihon Kohden, VC-7) and a inkwriting oscilloscope (Nihon Kohden, WI-180).