Intracellular Na+ measurements in smooth muscle using SBFI--changes in [Na+], Ca2+ and force in normal and Na(+)-loaded ureter.
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.
Lamont
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C
Veterinary Preclinical Sciences, The University of Liverpool, Liverpool, UK.
Burdyga
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TV
Wray
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D000818:Animals; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism*; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; D006168:Guinea Pigs; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D009130:Muscle, Smooth / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D014513:Ureter / Q000378:metabolism*
D001572:Benzofurans; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; C061665:sodium-binding benzofuran isophthalate; D012964:Sodium; D002118:Calcium