Abstract
Cardiomyocyte is known to hardly swell during the Na/K pump block, although it functions to maintain cell volume. Previously we analyzed the mechanism using a comprehensive mathematical model of cardiomyocyte, the Kyoto model, and proposed that plasma membrane Ca pump, reverse mode of Na/Ca exchanger (NCX) and window current of L-type Ca channel are the key factors responsible for the cardiac cell volume regulation. In the present study, we validated the hypothesis by conducting new experiments measuring [Na]i (monitored by SBFI), cell area and membrane potential (monitored by di-8-ANEPPS), with pharmacological interventions. When an NCX blocker 1 μM SEA0400 was applied to the cardiomyocyte after 60 min treatment with 40 μM ouabain, [Na]i started to increase, indicating the Na extrusion via reverse mode of NCX under the Na/K pump block condition. In addition, this drug caused a significant cell swelling, confirming the proposed mechanism that NCX in reverse mode prevents the cell swelling. When exposed to 1 μM isoproterenol, cardiomyocytes swelled after a delay, which was predicted to be triggered by L-type Ca channel window current. Additionally, application of an L-type Ca channel blocker 5 μM nifedipine eliminated the swelling almost completely. These experimental findings strongly supported the model prediction that the Ca transporters and channels take key roles in the cardiac cell volume regulation under the condition of Na/K pump block. [J Physiol Sci. 2007;57 Suppl:S80]
