Influence of internal calcium stores on calcium-activated membrane currents in smooth muscle

Agonist-induced activation of second messenger systems that increase inositol 1,4,5-trisphosphate concentration plays an important role in the mobilisation of stored Ca2+ in smooth muscle. Release of Ca2+ which occurs spontaneously or by agonist stimulation causes activation of Ca(2+)-sensitive currents. Experiments using selective modulators of the Ca2+ uptake and release mechanisms of the sarcoplasmic reticulum provide evidence that depletion of the internal Ca2+ store triggers an influx of Ca2+ from the extracellular space. Recent data suggest that pool depletion causes the opening of non-selective cation channels which are possibly similar to those previously designated 'receptor-operated Ca2+ channels'. Ca2+ entry through these channels not only refills the internal Ca2+ pool, but also modulates membrane excitability by stimulating Ca(2+)-activated currents.