Inositol 1,4,5‐trisphosphate activates pharmacomechanical coupling in smooth muscle of the rabbit mesenteric artery.

Abstract

To clarify the nature of the noradrenaline (NA)‐induced contraction, the effects of NA on inositol phospholipid metabolism and the actions of inositol 1,4,5‐trisphosphate (InsP3) on skinned muscle of the rabbit mesenteric artery were investigated. NA, in concentrations over 1 nM, reduced the amount of phosphatidylinositol 4,5‐bisphosphate (PI‐P2) and increased the amount of phosphatidic acid (PA). The maximum reduction in the amount of PI‐P2 and the maximum increase in the amount of PA were observed in the presence of 1 microM‐NA. With prolonged application of NA, the PI‐P2 was gradually restored to near the control level, but with repeated applications of NA separated by rinses with Krebs solution, there was a consistent reduction of PI‐P2. The NA‐induced PI‐P2 breakdown was inhibited by the alpha 1‐adrenoceptor blocking agent, prazosin. After incubation of the tissue with radioactive inositol‐containing solution, NA transiently increased the amount of radioactive InsP3 which was followed by increases in the amount of inositol 1,4‐bisphosphate (InsP2) and inositol monophosphate (InsP). After accumulation of Ca by saponin‐treated muscle cells of the dog mesenteric artery dispersed by collagenase, InsP3 released Ca stored in cells but InsP2 did not. A23187 (5 microM) but not InsP3 (up to 10 microM), depleted Ca accumulated in the presence of ATP. In saponin‐treated skinned muscle tissues, InsP3 in concentrations over 0.3 microM, produced contraction following accumulation of Ca into the store site. InsP3 released Ca from the same source as caffeine. The release of Ca by InsP3 appeared in a concentration‐dependent manner and this release also depended on the amount of Ca stored in cells (the median effective dose (ED50) was 3.0 microM in 0.1 microM‐Ca and 1.0 microM in 0.3 microM‐Ca). We concluded that NA activates alpha 1‐adrenoceptors, thus hydrolysing PI‐P2 and synthesizing InsP3. This product can release Ca stored in cells as estimated from the contraction in skinned muscle tissues, and also reduces the residual amount of Ca stored in skinned dispersed muscle cells. Contraction evoked by NA through pharmacomechanical coupling can be explained as a function of InsP3.