Calcium-dependent slow action potentials in potassium-depolarized guinea-pig ventricular myocardium enhanced by barium ions

Experiments were performed to investigated the mechanism by which Ba ions facilitate slow response in the myocardium. In partially depolarized guinea-pig papillary muscles, adding 0.2 mmol/l Ba to K-rich solution drastically lowered the stimulus threshold and converted the graded response to an all-or-none activity. This change was accompanied by an increase in the membrane resistance, as determined by cable analysis. When [Ca]0 was altered (0.9--7.2 mmol/l) in the presence of 0.2 mmol/l Ba, the action potentials varied almost in a manner expected for a Ca-electrode, and there were concomitant increases in the twitch tension. The electrical and mechanical activities of muscles treated with 1 mmol/l Ba were also strongly Ca-dependent. At normal [Ca]0, increasing [Ba]0 from 0.2--1 mmol/l only slightly enhanced the action potentials without any appreciable change in the peak twitch tension. When [Ca]0 was lowered to 0.2 mmol/l, elevation of [Ba]0 to over 0.5 mmol/l restored the action potentials, and the resting tension increased. These results suggest that the facilitatory action of the low concentration of Ba on the Ca-dependent slow action potentials is due to a reduction in the membrane shunting conductance and not to a development of any sizeable inward Ba current. However, Ba-dependent action potentials may be generated under conditions of Ca-deficiency. Isoprenaline (0.5--5 x 10(-7) mol/l) induced an automatic activity in the Ba-treated muscles and this phenomenon is probably related to both the drug-induced increase in Ca-permeability and the Ba-induced decrease in shunting conductance. Thus Ba ions appear to be a feasible tool for studies on the myocardial slow channel.

Interaction between caffeine and adenosine on the membrane current and tension component in the bullfrog atrial muscle

Antagonistic action of caffeine and adenosine were studied at the membrane current and tension component levels in myocardium of the bullfrog atrium, mainly under voltage clamp using the double sucrose-gap method. Caffeine (1-5 mM) produced marked augmentation of the phasic and tonic tensions as well as slow inward current (Is). At higher concentrations it elicited an increase of the delayed outward current (Ix). The augmentation of Is was principally due to increase of the limiting conductance (gs), while the activation and inactivation variables (dinfinity, finfinity) were not changed significantly. The dose-tension response curve for caffeine, which appeared sigmoidal, was notably lowered in the presence of adenosine (1-3 mM), indicating a competitive type of inhibition. Adenosine, in the presence of caffeine, exerted a narcotic-like action by inhibiting all of the membrane currents (INaf, Is, Ix and background inward current, Ib) and tension components (ICa-dependent and independent tensions). The inhibition of Is was due to a decrease of gs and no appreciable change was observed in dinfinity and finfinity. These results suggest that adenosine has a strong stabilizing action on the myocardium, especially when the heart muscle activity is accelerated by increased cyclic AMP.

The voltage- and time-dependent effects of (-)-verapamil on the slow inward current in isolated cat ventricular myocardium

The effects of (-)-verapamil on the slow inward current in cat papillary muscle were investigated under voltage clamp conditions. The following results, which correlate significantly with the negative inotropic effect of the drug, were obtained: (-)-verapamil reduced the slow inward current in a highly frequency dependent manner, with greater reduction at higher frequency; at cycle intervals longer than 10 sec, the inhibition practically disappeared. As an associated phenomenon there was a stepwise decrease of slow inward current subsequent to an increase in frequency. This "staircase" of the current was facilitated with lowering the holding potential and was diminished by hyperpolarization. Possible mechanisms underlying the membrane action of (-)-verapamil are discussed.