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¹ Smooth Muscle Research Centre, Dundalk Institute of Technology, Dundalk, County Louth, Ireland

Measurements were made (using fast confocal microscopy) of intracellular Ca²⁺ levels in fluo-4 loaded interstitial cells isolated from the rabbit urethra. These cells exhibited regular Ca²⁺ oscillations which were associated with spontaneous transient inward currents recorded under voltage clamp. Interference with D-myo-inositol 1,4,5-trisphosphate (IP₃) induced Ca²⁺ release using 100 µM 2-aminoethoxydiphenyl borate, and the phospholipase C (PLC) inhibitors 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate and U73122 decreased the amplitude of spontaneous oscillations but did not abolish them. However, oscillations were abolished when ryanodine receptors were blocked with tetracaine or ryanodine. Oscillations ceased in the absence of external Ca²⁺, and frequency was directly proportional to the external Ca²⁺ concentration. Frequency of Ca²⁺ oscillation was reduced by SKF-96365, but not by nifedipine. Lanthanum and cadmium completely blocked oscillations. These results suggest that Ca²⁺ oscillations in isolated rabbit urethral interstitial cells are initiated by Ca²⁺ release from ryanodine-sensitive intracellular stores, that oscillation frequency is very sensitive to the external Ca²⁺ concentration and that conversion of the primary oscillation to a propagated Ca²⁺ wave depends upon IP₃-induced Ca²⁺ release.

(Received 26 October 2004; accepted after revision 7 March 2005; first published online 10 March 2005)
Corresponding author N. McHale: Smooth Muscle Research Centre, Dundalk Institute of Technology, Dundalk, County Louth, Ireland. Email: n.mchale{at}smoothmusclegroup.org

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