Calcium release from intracellular stores and excitation-contraction coupling in intestinal smooth muscle

Calcium release from intracellular stores plays a central role in excitation-contraction coupling of striated and smooth muscle cells. Two main intracellular calcium pools have been identified in phasic smooth muscle: (1) the inositol 1,4,5-trisphosphate-sensitive and the (2) ryanodine-sensitive calcium stores. We studied the contribution of the ryanodine-sensitive calcium stores to the excitation-contraction coupling in the intestine. The intracellular calcium concentration was measured in cultured intestinal smooth muscle cells using the fluorescent probe fura-2-AM. Isometric tension generated by the murine jejunum was recorded in vitro using force displacement transducers. The cytosolic calcium level increased significantly on cholinergic stimulation. The rise persisted in the absence of extracellular calcium. Depletion of ryanodine-sensitive calcium stores with caffeine or ryanodine blunted the response to a cholinergic agonists. Similarly, the ryanodine receptor channel blocker dantrolene significantly decreased the carbachol-induced calcium increase. We subsequently tested the effects of these pharmacological tools on the spontaneous and carbachol-induced contractions of the murine jejunum. Depletion of the ryanodine-sensitive stores and calcium release channel block both significantly decreased the contractile activity of the circular and longitudinal layer of the muscularis propria. Our data confirm the importance of intracellular calcium stores in excitation-contraction coupling of intestinal smooth muscle cells. The effects of different pharmacological tools on the intracellular calcium signal and the contractile function are consistent with other observations in phasic smooth muscle. They suggest a significant contribution of calcium release from ryanodine-sensitive stores to the calcium signal that triggers contraction.