The role of Ni2+-sensitive T-type Ca2+ channels in the regulation of spontaneous excitation in detrusor smooth muscles of the guinea-pig bladder

Authors


Yoshimasa Yanai, Department of nephro-urology, Nagoya City University Medical School, Nagoya 467–8601, Japan.
e-mail: y-yanai@med.nagoya-cu.ac.jp

Abstract

OBJECTIVE

To explore the role of Ni2+-sensitive T-type Ca2+ channels in the generation of spontaneous excitation of detrusor smooth muscles.

MATERIALS AND METHODS

In isolated detrusor smooth muscle bundles of the guinea-pig bladder, changes in the membrane potential and muscle tension were measured using intracellular microelectrodes and isometric tension recording. Changes in the intracellular Ca2+ concentration were recorded from bundles loaded with the fluorescent dye fura-PE3.

RESULTS

Detrusor smooth muscles had two types of spontaneous electrical activity, i.e. individual and bursting action potentials. Ni2+ (30 µm), a blocker for T-type Ca2+ channels, reduced the frequency of individual action potentials without changing their amplitude. Higher concentrations of Ni2+ (100–300 µm) converted individual action potentials into the bursts, as did apamin (0.1 µm), a blocker of small-conductance Ca2+-activated K+ channels (SK). They also increased the amplitudes of spontaneous Ca2+ transients and corresponding contractions whilst reducing their frequencies. In preparations which generated bursting action potentials, nifedipine (1 µm) converted action potentials into spontaneous transient depolarizations (STDs), and subsequent applications of Ni2+ (100 µm) abolished STDs. Gadolinium (100 µm) and SKF96365 (10 µm), blockers for nonselective cation channels, and niflumic acid (100 µm), a blocker for Ca2+-activated Cl– channels, had no effect on either the amplitude or frequency of spontaneous action potentials.

CONCLUSIONS

The T-type Ca2+ channel may have dual roles in generating spontaneous excitation in detrusor smooth muscles. First, activity of these channels may account for the preceding depolarizations that lead to action potentials. Second, Ca2+ influx through T-type Ca2+ channels may couple functionally to SK channels, contributing to the stability of the resting membrane potential in detrusor smooth muscle. Thus, pharmacological manipulation of T-type Ca2+ channels in detrusor smooth muscles could be of potential value for treating the overactive bladder.

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