Inositol trisphosphate has no direct effect on the contractile apparatus of skinned cardiac muscles

Alpha-adrenoceptor stimulation produces a positive inotropic effect in heart muscle via mechanisms that are not well understood. The purpose of our study was to test the hypothesis that the increase in inositol 1,4,5 trisphosphate [Ins(1,4,5)P3] concentration that accompanies alpha stimulation contributes to the inotropic effect by increasing the calcium sensitivity of the contractile proteins, an effect which Ins(1,4,5)P3 has been shown to have in skeletal muscle. We determined the calcium sensitivity of the contractile apparatus of small, chemically skinned bundles from papillary muscles of rabbit, rat and dog hearts. These preparations were chosen because they exhibit a range of sensitivity to alpha agonists. In addition, we measured the calcium sensitivity of chemically skinned, single fibers from rabbit psoas muscle. All preparations were skinned with Triton X-100, a non-ionic detergent that disrupts the sarcolemmal, sarcoplasmic reticular, and mitochondrial membranes. In all cardiac preparations, we found that 38 microM Ins(1,4,5)P3 had no effect on either the calcium sensitivity or maximum calcium-activated force. Thus, there was no correlation between inotropic response to alpha stimulation and myocardial response to Ins(1,4,5)P3. On the other hand, the maximum calcium-activated force of skinned skeletal muscle was slightly increased by Ins(1,4,5)P3. Moreover, Ins(1,4,5)P3 significantly increased the sensitivity of these fibers to calcium.