The influence of changes in extracellular and intracellular sodium concentration on detrusor contractility.

Abstract

OBJECTIVE:

To determine the role of Na+-Ca2+ exchange in the regulation of isolated detrusor smooth muscle contractility.

MATERIALS AND METHODS:

Isolated guinea-pig detrusor strips were used to record isometric tension generated by; (a) electrical-field stimulation to elicit nerve-mediated responses; and (b) adding carbachol or superfusing with a high-K+ solution. The [Na+] gradient between extracellular and intracellular compartments was altered by: (i) reducing superfusate [Na+] in stages from 140.2 to 10.2 mm; (ii) addition of the cardiac glycoside strophanthidin (200 microm). RESULTS Reducing extracellular [Na+] reversibly reduced the magnitude of nerve-mediated contractions but increased the resting tension and magnitude of carbachol-induced contracture. The mean (sd) [Na+] required for a half-maximum effect on attenuating contractions, at 85.9 (6.2) mm, and developing contracture, at 59.1 (14.3) mm, were significantly different. The time constants of changes to nerve-mediated contractions and carbachol contracture were also significantly different, at 147 (5) vs 1207 (386) s, respectively. These differences suggest that separate mechanisms influence nerve-mediated contraction and contracture in low-Na+ solutions. Exposure to the cardiac glycoside strophanthidin produced a similar effect to low-Na+ solutions for carbachol contracture. Low-Na+ solutions had no significant effect on contractures induced by high extracellular [K+]. CONCLUSION Reducing the transmembrane [Na+] difference increases intracellular [Ca2+]. This increase is largely accommodated in intracellular stores, that can be released by exogenous carbachol. The results are consistent with the presence of a functional Na+-Ca2+ exchanger in the surface membrane. The lack of effect of low-Na+ solutions on contractures evoked by membrane depolarization is consistent with this conclusion. The reduction of the nerve-mediated contraction by low-Na+ solution might result from blockade of the nerve action potential.

• Authors: Dler Besarani, Changhao Wu, Christopher H Fry
• Journal: BJU international (BJU Int) Vol. 97 Issue 5 Pg. 1083-6 (May 2006) ISSN: 1464-4096 [Print] England
• PMID: 16643496 (Publication Type: Journal Article)

Chemical References

• Sodium-Calcium Exchanger
• Carbachol
• Sodium
• Calcium

Topics

• Animals
• Calcium (metabolism)
• Carbachol (pharmacology)
• Electric Stimulation
• Extracellular Fluid (metabolism)
• Guinea Pigs
• Intracellular Fluid (metabolism)
• Isometric Contraction (drug effects)
• Muscle Contraction (physiology)
• Muscle, Smooth (drug effects, innervation, metabolism)
• Sodium (metabolism, pharmacology)
• Sodium-Calcium Exchanger (drug effects, physiology)
• Urinary Bladder (drug effects, physiopathology)