Sodium/calcium exchange in mammalian ventricular muscle: a study with sodium‐sensitive micro‐electrodes.

Abstract

A method for mounting and rapidly perfusing small ventricular trabeculae (diameter around 250 .mu.m) from either ferret or guinea pig is described. Tension, membrane potential (Em) and intracellular Na activity .**GRAPHIC**. was measured. .**GRAPHIC**. was measured using Na-sensitive micro-electrodes. At room temperature (22.degree.-26.degree. C), [Na]o 155 mmol/l and [Ca]o 5.4 mmol/l, .**GRAPHIC**. was 10.9 mmol/l .+-. SD 4.2 mmol/l (n = 148). When [Na]o was reduced from 155 to 1.5 mmol/l contractures developed. These were about twitch height in guinea pig but less than the twitch height in ferret. Associated with the development of the contracture there was a decrease in .**GRAPHIC**. The .**GRAPHIC**. halved within 30 s. The decrease in .**GRAPHIC**. was not influenced by changing pHo from 7.4 to 9.5, K-free solution or strophanthidin 50 .mu.mol/l and was not passive since, even when the [Na]o was 1.5 mmol/l, the driving force for Na ions remained inward. The .**GRAPHIC**. decreased if [Ca]o was increased and [Na]o decreased or vice versa. Evidently, the decrease in .**GRAPHIC**. is mainly due to Na/Ca exchange. Despite the large decrease in .**GRAPHIC**. the [Ca]i, as monitored by tension changes, hardly increased. Since Ca uptake does occur in Na-free conditions in heart muscle it is proposed that the mitochondria take up Ca ions and so prevent an excessive rise in cytoplasmic Ca. Strophanthidin increased both .**GRAPHIC**. and the withdrawal contracture, but collected results from a number of experiments showed no clear correlation between the initial .**GRAPHIC**. and contracture amplitude. Strophanthidin may, therefore, have actions additional to increasing .**GRAPHIC**.