Effects of Variations in Extracellular Osmolality on Spontaneous Contractile Activity and Response to Nerve Stimulation in Rat Detrusor Muscle in vitro

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Abstract
The effects of hyperosmolar (390, 590 mosm/kg) and hypoosmolar (260 mosm/kg) solutions on myogenic spontaneous contractile activity and response to nerve stimulation were studied on strips of rat detrusor muscle in vitro. In isotonic (290 mosm/kg) solution the mean spontaneous contractile activity was 2.1 ± 0.6% and the amplitude of the largest spontaneous contractions was 9.8 ± 3.0% of maximal response to nerve stimulation. In 390 mosm/kg the frequency of spontaneous contractions decreased, but due to a fourfold increase of contraction amplitude a threefold increase of mean contractile activity was recorded. In 260 mosm/kg a similar contraction pattern as in 290 mosm/kg was seen. The spontaneous contractions were resistant to tetrodotoxin (10––5 g/ml) in all osmolalities. In 590 mosm/kg the spontaneous activity had ceased, and a continuous contracture (resistant to tetrodotoxin) was observed with an amplitude of 41 ± 1 % of the maximal response to nerve stimulation at 290 mosm/kg. Similar maximal responses to nerve stimulation were obtained in 260, 290, and 390 mosm/kg. The frequency-response relation in 390 mosm/kg had a leftward shift at low frequencies. In 590 mosm/kg no response to nerve stimulation or acetylcholine was noted, but the preparations contracted vigorously following depolarization with high-K+ solution.