The actions of altered osmolarity on guinea-pig detrusor smooth muscle contractility and intracellular calcium

The study measured the effects in vitro of changing extracellular osmolarit y on the contractility of detrusor smooth muscle strips. The data were inte rpreted in the context of separate measurements from isolated cells of alte rations to the intracellular [Ca2+], [Ca2+](i). Increased osmolarity (300-7 00 mosmol 1(-1)) reduced phasic contractions but increased resting tension regardless of whether sucrose, LiCl or NaCl were used as osmolytes. [Ca2+]( i) was decreased slightly only when NaCl increased osmolarity, otherwise it was unchanged. The contractile effects may be explained by tissue shrinkag e and reduction of detrusor excitability. Lowered osmolarity (300-64 mosmol 1(-1)) decreased phasic contractions but increased resting tension and [Ca 2+],. The raised resting tension was due solely to low osmolarity and was i ndependent of changes to [Na], [Cl] or ionic strength. The rise of [Ca2+](i ) was due partly to Ca2+ influx through Na+ Ca2+ exchange but a fraction wa s independent of extracellular Ca, unaffected by Gd3+, and persisted in the presence of caffeine. By contrast, reduction of phasic tension was due mai nly to the reduced ionic strength, not osmolarity. The results do not suppo rt the presence of functional stretch-activated channels and suggest only a minor role for Na+-Ca2+ exchange under these conditions. However, they do suggest an intracellular source of Ca2+, which is independent of the sarcop lasmic reticulum.