Properties of Ca(2+)-mediated inactivation of L-type Ca channel in smooth muscle cells of the guinea-pig urinary bladder

Ionic basis for the regulation of spontaneous excitation in detrusor smooth muscle cells of the guinea-pig urinary bladder

(1) The regulatory mechanisms of spontaneous excitation in detrusor smooth muscles of the guinea-pig urinary bladder were investigated using intracellular microelectrode and muscle tension recording techniques. (2) Detrusor smooth muscle cells exhibited nifedipine-sensitive spontaneous action potentials. Their frequency was highly sensitive to membrane polarization and was reduced by lowering the temperature. Lowering the temperature also reduced the frequency of spontaneous contractions and increased their amplitude. (3) Charybdotoxin (50 nm) and iberiotoxin (0.1 microm) increased the amplitude and duration of action potentials, and abolished after hyperpolarizations (AHPs). Both agents also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. Apamin (0.1 microm) did not change the shape of action potentials but often converted individual action potentials into bursts. It also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. 4-aminopyrideine (4-AP, 1 mm) increased the frequency of action potentials without affecting their shape, and increased the amplitude and frequency of spontaneous contractions. (4) Cyclopiazonic acid (CPA, 10 microm) and ryanodine (50 microm) increased the amplitude of action potentials, and suppressed AHPs. Both agents also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. 1,2-(Bis (2-aminophenoxy) ethane-N,N,N', N'-tetraacetic acid tetrakis (acetoxymethyl ester) (50 microm) dramatically increased the amplitude and duration of the action potential, and abolished AHPs. (5) Spontaneous action potentials in detrusor smooth muscles cells result from the opening of L-type Ca2+ channels, and their frequency is regulated by voltage-dependent mechanisms and by some metabolic process. Both the activation of large conductance Ca2+-activated K+ (BK) channels and Ca2+-mediated inactivation of the Ca2+ channels are involved in the repolarizing phase of action potentials. The Ca2+ influx through L-type Ca2+ channels triggers calcium-induced calcium release via ryanodine receptors and activates BK channels to generate AHPs. Both small conductance Ca2+-activated K+ channels and voltage-sensitive K+ channels may contribute to the resting membrane potential and regulate the frequency of action potentials. The regulatory mechanisms of action potentials are closely related to the regulation of spontaneous contractions.

Electrophysiological properties of the bladder

The electrophysiological properties of detrusor smooth muscle are described, in particular with regard to their influence on the contractile properties of the tissue. The Ca2+ and K+ channel activities are most important in generating action potentials, but the role of several other ionic currents is described, including Cl-, Ca2+-activated, stretch-activated and ligand-gated channels. The variable appearance and functions of different ionic currents in disease states is discussed, as well as the question of whether electrical activity can transmit between adjacent smooth muscle cells. In addition, the precise role that electrophysiological phenomena play in the regulation of the contractile state of the smooth muscle cells, as well as the generation of bladder electromyograms, is discussed.

Voltage-dependent suppression of calcium current by caffeine in single smooth muscle cells of the guinea-pig urinary bladder

The suppressive action of caffeine on L-type Ca current (Ica) in smooth muscle cells of the guinea-pig urinary bladder was investigated using the whole-cell patch clamp technique. Caffeine (5-30 mM) suppressed Ica, the effect having two phases: a rapid and transient suppression of Ica, which was followed by a sustained suppression. When intracellular Ca2+ was strongly buffered by the Ca2+ chelator EGTA (20 mM) or BAPTA (5 mM) in the patch pipette, the transient suppression of Ica was abolished, whereas the sustained effect remained. Similarly, inclusion of both 10 mM procaine and 1 mg/ml heparin in the patch pipette blocked the transient suppression of Ica, but did not block the sustained effect. The degree of the sustained effect of caffeine on Ica was dose-dependent with a kd of 20 mM. Application of the cyclic AMP analogue, 8-bromo-cyclic AMP (100 microM) or forskolin (10 microM) to the bath failed to mimick the sustained suppression of Ica, suggesting that inhibition of phosphodiesterase activity was not involved in the caffeine action. The steady-state activation curve remained unchanged by 10 mM caffeine but the steady-state inactivation curve was significantly shifted in the negative direction by 15.6 mV in 1.8 mM Ca2+ solution or by 10 mV in 1.8 mM Ba2+ solution. From these results it appears that caffeine inhibits L-type Ica via two mechanisms: (1) it releases Ca2+ from an internal store causing a transient Ca2+ -mediated inactivation of the Ca channel; (2) it inhibits Ca channel via a mechanism that does not require such a Ca2+ release. It is possible that caffeine suppresses Ica through a preferential binding to the inactivated state of L-type Ca channel.