A quantitative analysis of the Na+-dependence of Vmax of the fast action potential in mammalian ventricular myocardium. Saturation characteristics and the modulation of a drug-induced INa blockade by [Na+]o.

Abstract

In microelectrode experiments on papillary muscles of guinea pigs, the quantitative dependence of\(\dot V_{{\text{max}}} \) of the fast action potential, taken as a measure of INa, on external Na+ concentration has been analyzed under different experimental conditions including the presence of antiarrhythmic drugs such as lidocaine, procaine and propafenone. 1. External Na+ concentration changes between 225 mmol/l and 45 mmol/l led to a non-linear response of\(\dot V_{{\text{max}}} \) in that the\(\dot V_{{\text{max}}} \) changes obtained experimentally were significantly smaller than predicted theoretically from a linear dependence of\(\dot V_{{\text{max}}} \) on [Na+]o. 2. Each individual\(\dot V_{{\text{max}}} \) relationship exhibited saturation characteristics. In Lineweaver-Burk plots,\(\dot V_{{\text{max}}} \) correlated extremely well to 1/[Na+]o with correlation coefficients between 0.995 and 0.999. In 16 experiments, the apparentKm for Na+ varied within a range from 170 to 455 mmol/l. Values of 450 V/s–900 V/s were calculated for the saturated\(\dot V_{{\text{max}}} \) (at an infinitely large [Na+]o). 3. The apparentKm for Na+ rose from 232.0±24.7 mmol/l to 544.0±50.2 mmol/l when the K+ concentration of the medium was increased from 5.4 to 10 mmol/l and, thus, resting potential declined from −90.5±2.5 mV to −76.1±1.8 mV. 4. Alkalization (pH 9.0) of the medium lowered the apparentKm for Na+ and, simultaneously, reduced the saturated\(\dot V_{{\text{max}}} \). This typical shift of the straight relating\(\dot V_{{\text{max}}} \) to 1/[Na+]o in the Lineweaver-Burk plot excludes a competitive interaction between Na+ and H+ ions. 5. Lidocaine (5×10−5–2×10−4 mol/l), procaine (2×10−4 mol/l) and propafenone (0.5–3×10−5 mol/l) depressed\(\dot V_{{\text{max}}} \) the stronger the lower [Na+]o was. The changes of the\(\dot V_{{\text{max}}} \) relationship induced by these drugs indicated neither a competitive nor a non-competitive interaction of these compounds with Na+. 6. As tested with propafenone, external Na+ changes modulated the tonic and the phasic block of\(\dot V_{{\text{max}}} \). The Na+ sensitivity of both types of block differed considerably. In a Na+-poor (50 mmol/l) medium, the apparentKm for the tonic block declined from 5×10−5 to 1.4×10−5 mol/l and the apparentKm for the phasic block from 3.8×10−5 to 2.3×10−5 mol/l. 7. Na+ is not the sole cation that determines the strength of a drug-induced blockade of\(\dot V_{{\text{max}}} \) as it can be substituted by the permeant Li+. 8. In the presence of drugs like propafenone which are capable of shifting the steady state inactivation (h∞) of INa to more negative potentials, the voltage-dependence ofh∞ can be modified by external Na+ variations. Na+ withdrawal led to a considerable shift in the hyperpolarizing direction.