InsP3-dependent Ca2+ mobilization in circular but not longitudinal muscle cells of intestine

The presence and functional coupling of inositol 1,4,5-trisphosphate (InsP3) receptors was examined in permeabilized muscle cells and microsomes isolated separately from the circular and longitudinal muscle layers of guinea pig intestine. Specific binding of [3H]InsP3 occurred only in circular muscle cells and microsomes; binding to longitudinal muscle cells or microsomes was minimal or absent. Binding to circular muscle cell microsomes was rapid (steady state in 2 min), saturable, reversible, and temperature independent, and was inhibited in a concentration-dependent manner by unlabeled InsP3 (IC50 10 nM) and by the InsP3 receptor antagonist heparin (EC50 4 micrograms/ml); inhibition by InsP4 and InsP2 was minimal. Consistent with the preferential location of InsP3 receptors, InsP3 elicited concentration-dependent contraction (EC50 3 nM), 45Ca2+ efflux (EC50 55 nM), and increase in [Ca2+]i in circular muscle cells and 45Ca2+ efflux in microsomes; all these events were blocked by heparin (10 micrograms/ml). Although ATP-dependent Ca2+ stores were present in longitudinal muscle cells and microsomes, they were not sensitive to 1 microM InsP3, which was maximally effective in circular muscle cells; 100 microM InsP3 elicited partial Ca2+ release (30-50%) and contraction (47%) that could only be blocked by high concentrations of heparin (100 micrograms/ml). We conclude that specific, high-affinity, heparin-sensitive InsP3 receptors capable of mobilizing intracellular Ca2+ are present predominantly in circular muscle cells of the intestine.