Sodium/calcium exchange and intracellular calcium buffering in ferret myocardium: an ion‐sensitive micro‐electrode study.

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Abstract
Measurements of the intracellular activity of Ca .**GRAPHIC**. Na .**GRAPHIC**. and H (pHi) ions have been made with resin-filled ion-sensitive micro-electrodes in ferret ventricular trabeculae. The mean values in quiescent muscle at 30.degree. C were: .**GRAPHIC**. 11.1 .+-. 1.0 mM; .**GRAPHIC**. 58.4 .+-. 6.4 nM, and pHi, 7.20 .+-. 0.11. The relation between .**GRAPHIC**. and extracellular Na activity .**GRAPHIC**. is not linear and is sensitive to temperature: the Q10 for the change in .**GRAPHIC**. in normal Tyrode solution is 1.3 .+-. 0.5 and rises to 3.5 .+-. 0.5 when .**GRAPHIC**. is reduced to1.1 mM. The addition of CN to the bathing fluid causes little or no change in .**GRAPHIC**. or .**GRAPHIC**. but pHi rises to 7.38 .+-. 0.10, yet in some preparations resting tension increases. Similar results are seen with carbonyl cyanide m-chlorophenyl hydrazone. On lowering [Na]o, the fall in .**GRAPHIC**. is very much greater than the rise in .**GRAPHIC**. and the pHi is generally unchanged. When [Na]o is lowered in the presence of a respiratory inhibitor, the fall in .**GRAPHIC**. is reduced, the rise in .**GRAPHIC**. and the contracture tension are increased while pHi falls. The apparent coupling ratio for the Na/Ca exchange varies between 3 and 4 depending on the experimental conditions. These results suggest that an intracellular process, with a high Q10 and which depends upon respiration and .**GRAPHIC**. is able to remove Ca2+ from the sarcoplasm and thereby interact with the sarcolemmal Na/Ca exchange. This process could be the increase in the energy-dependent accumulation of Ca2+ within mitochondria that will occur when the Ca efflux from these organells is progressively inhibited as .**GRAPHIC**. falls.