Mechanism of action of endothelin in rat cardiac muscle: cross‐bridge kinetics and myosin light chain phosphorylation

Abstract

The molecular mechanism of inotropic action of endothelin was investigated in rat ventricular muscle by studying its effects on characteristics of isometric twitch, barium‐induced steady contracture and the level of incorporation of 32Pi into myosin light chain 2. Exposure of rat papillary muscle to endothelin caused an increase in isometric twitch force but did not alter the twitch‐time parameters. Endothelin did not significantly change the maximum contracture tension but did cause an increase in contracture tension at submaximal levels of activation, without changes in the tension‐to‐stiffness ratio and kinetics of attached cross‐bridges. Kinetics of attached cross‐bridges were deduced during steady contracture from complex‐stiffness values, and in particular from the frequency at which muscle stiffness assumes a minimum value, fmin. Endothelin did not alter fmin. Endothelin caused an increase in the level of incorporation of 32Pi into myosin light chain 2 without a concurrent change in the level of incorporation of 32Pi into troponin I. We conclude that the inotropic action of endothelin is not due to an increase in the kinetics of attached cross‐bridges, nor due to a change in the force per unit cross‐bridge, but may result from an increased divalent cation sensitivity caused by elevated myosin light chain 2 phosphorylation, resembling post‐tetanic potentiation in fast skeletal muscle fibres.

Wortmannin Inhibits the Increase in Myofilament Ca2+ Sensitivity Induced by Cross-Talk of Endothelin-1 With Norepinephrine in Canine Ventricular Myocardium

“…The present results indicate that the mechanism of myofilament Ca 2+ sensitization induced by ET-1 is identical in that activation of MLCK may be involved in both species, which is susceptible to wortmannin at a concentration of 1 μM. These findings are consistent with those of Rossmanith et al that the PIE of ET-1 may result from an increased Ca 2+ sensitivity related to an increase in MLC-2 phosphorylation in cardiac muscle (10).…”

“…It has been shown that ET-1, angiotensin II (AT 1 -receptor stimulation), and phenylephrine (α 1 -adrenoceptor stimulation) share the common signaling pathway coupled to heterotrimeric G q proteins resulting in activation of PKC and MLCK in the heart (10,15,16). Namely, activation of G q proteins leads to generation of DAG that stimulates PKC activity to lead to phosphorylation of both troponin I (13,23,24) and MLC-2 (13,23) in cardiac myocytes.…”

“…Furthermore, the PIE of cardiac α 1 -adrenoceptor stimulation that induces an increase in Ca 2+ sensitivity in cardiac muscle is associated with the MLC-2 phosphorylation (15,16). ET-1 increases likewise MLC-2 phosphorylation in rat cardiac muscle (10).…”

“…sensitivity induced by ET-1 is due to an intracellular alkalinization and / or an increase in phosphorylation of myofilaments (10,11), but the signaling process responsible for the Ca 2+ sensitization has not been fully clarified.…”

Biochemical and Physiological Regulation of Cardiac Myocyte Contraction by Cardiac-Specific Myosin Light Chain Kinase

“…24 Other neurohumoral stimulators, such as endothelin 50 and prostanoid F receptors, 51 have also been reported to increase MLC2 phosphorylation and induce a prominent increase in the contractile force in the heart. Neuregulin, which activates ErbB receptor tyrosine kinase, has been reported to enhance the expression of cMLCK with a concomitant increase in MLC2v phosphorylation and improved cardiac performance after myocardial infarction in rats.…”