Augmented agonist-induced Ca2+-sensitization of coronary artery contraction in genetically hypertensive rats: Evidence for altered signal transduction in the coronary smooth muscle cells

Abstract

The Ca2+ responsiveness of vascular smooth muscle myofilaments is not unique: it is increased during neuro-humoral activation and decreased during β-adrenergic stimulation. In this study we tested whether an augmented Ca2+ responsiveness of smooth muscle myofilaments may contribute to the increased coronary tone observed in hypertension using β-escin-permeabilized coronary arteries from 3-mo-old stroke-prone spontaneously hypertensive rats (SHRSP) and their age matched normotensive reference strain (WKY rats). In intact coronary arteries, the response to 5-hydroxytryptamine (5-HT) but not to KCl was larger in SHRSP than in WKY rats. In β-escin permeabilized coronary arteries in which the receptor effector coupling is still intact, 5-HT enhanced force at constant submaximal (Ca2+) (pCa 6.38) to a greater extent in SHRSP. The Ca2+ sensitizing effect of 5-HT was mimicked by GTPγS (0.01-10 μM); again this effect was larger in SHRSP. In the absence of 5-HT or GTPγS the Ca2+ force relation was similar in both groups. Forskolin induced relaxation at constant submaximal (Ca2+). This desensitizing effect was smaller in SHRSP than in WKY rats. In conclusion, this study shows that intracellular signalling pathways involved in modulating the Ca2+ responsiveness of coronary smooth muscle myofilaments are altered in the genetically hypertensive animals favoring a hypercontractile state in the coronary circulation.