Evidence that a Ca 2+ sparks/STOCs coupling mechanism is responsible for the inhibitory effect of caffeine on electro-mechanical coupling in guinea pig ureteric smooth muscle

Abstract

Recent studies have highlighted the role of the sarcoplasmic reticulum (SR) in controlling excitability, Ca 2+ signalling and contractility in smooth muscle. Caffeine, an agonist of ryanodine receptors (RyRs) on the SR has been previously shown to effect Ca 2+ signalling but its effects on excitability and contractility are not so clear. We have studied the effects of low concentration of caffeine (1 mM) on Ca 2+ signalling, action potential and contractility of guinea pig ureteric smooth muscle. Caffeine produced reversible inhibition of the action potentials, Ca 2+ transients and phasic contractions evoked by electrical stimulation. It had no effect on the inward Ca 2+ current or Ca 2+ transient but increased the amplitude and the frequency of spontaneous transient outward currents (STOCs) in voltage clamped ureteric myocytes, suggesting Ca 2+-activated K + channels (BK) are affected by it. In isolated cells and cells in situ caffeine produced an increase in the frequency and the amplitude of Ca 2+ sparks as well the number of spark discharging sites per cell. Inhibition of Ca 2+ sparks by ryanodine (50 μM) or SR Ca 2+-ATPase (SERCA) cyclopiazonic acid (CPA, 20 μM) or BK Ca channels by iberiotoxin (200 nM) or TEA (1 mM), fully reversed the inhibitory effect of caffeine on Ca 2+ transients and force evoked by electrical field stimulation (EFS). These data suggest that the inhibitory effect of caffeine on the action potential, Ca 2+ transients and force in ureteric smooth muscle is caused by activation of Ca 2+ sparks/STOCs coupling mechanism.