Phosphoinositide-derived second messengers and the regulation of Ca²⁺ in vascular smooth muscle

Vascular smooth muscle and the phosphoinositide signalling pathway: Vascular smooth muscle tone can be regulated by an array of agonists which act via receptor-mediated transmembrane signalling pathways to modify the concentrations of key intracellular second messengers. Following agonist stimulation the phosphoinositide signalling pathway initiates the contraction process in vascular smooth muscle, via the second messengers myo-inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol. Diversity of calcium-regulatory mechanisms: The vascular smooth muscle cell apparently sustains the contraction with a number of diverse mechanisms, which act to increase intracellular Ca²⁺ by regulating both Ca²⁺ influx across the plasma membrane and Ca²⁺ release from intracellular calcium stores, or may act in the apparent absence of elevated cytosolic Ca²⁺ concentrations. Future research: The exact nature of these physiological interactions and their exact function are not yet fully understood. In particular, identification of the natural role of specific phospholipase C(δ), phospholipase C(γ) and protein kinase C isozymes and also the various ryanodine and myo-inositol 1,4,5-trisphosphate receptor subtypes present in vascular smooth muscle will prove critical to future understanding of the regulation of vascular smooth muscle tone in both the normal and the hypertensive phenotype.