Intracellular Na+ measurements in smooth muscle using SBFI--changes in [Na+], Ca2+ and force in normal and...

Our understanding of the control and effects of intracellular [Na+] ([Na+]i) in intact smooth muscle is limited by the lack of data concerning [Na+]i. The initial aim of this work was therefore to investigate the suitability of using the Na+-sensitive fluorophore SBFI in intact smooth muscle. We find this to be a good method for measuring [Na+]i in ureteric smooth muscle. Resting [Na+]i was found to be around 10 mM and rose to 25 mM when the Na+-K+-ATPase was inhibited by ouabain. This relatively low [Na+]i in the absence of Na+-K+-ATPase suggests that other cellular processes, such as Na+-Ca2+ exchange, play a role in maintaining [Na+]i under these conditions. Simultaneous measurements of [Na+]i or [Ca2+] i and force showed that Na+-Ca2+ exchange can play a functional role in ureteric smooth muscle. We found that the greater the driving force for Na+ exit and hence Ca2+ entry, the larger the contraction. In addition the Na+-Ca2+ exchanger activity under these conditions was found to be pH sensitive: acidification reduced the contraction and concomitant changes in [Ca2+] and [Na+]i. We conclude that SBFI is a useful method for monitoring [Na] in smooth muscle and that Na+-Ca2+ exchange may play a functional role in the ureter.