Protamine relaxes vascular smooth muscle by directly reducing cytosolic free calcium concentrations in small resistance arteries

Protamine has been suggested to relax vascular smooth muscle by reducing the intracellular Ca²⁺ concentration ([Ca²⁺]_i). However, there has been no direct evidence that protamine reduces the [Ca²⁺]_i of vascular smooth muscle. We therefore studied the effects of protamine on changes in [Ca²⁺]_i and tension induced by norepinephrine (NE) and high K⁺ in endothelium-denuded strips from rabbit small mesenteric artery, using fura-2-fluorometry and isometric tension recording methods. Both NE (1 μM) and high K⁺ (40 mM) produced a transient phasic increase, followed by a tonic increase in [Ca²⁺]_i and tension. Protamine concentration (15-500 μg·ml^-1)-dependently inhibited (P<0.05) the phasic and tonic components of both NE- and high K⁺-induced contraction with IC₅₀ values of ≈50 μg·ml^-1. Protamine (50 μg·ml^-1) inhibited (P<0.05) the phasic and tonic increases in [Ca²⁺]_i caused by both NE and high K⁺ by ≈40%-60%. We conclude that the direct vasodilator action of protamine is due, at least in part, to reduction of [Ca²⁺]_i in vascular smooth muscle; this reduction in [Ca²⁺]_i may be due to inhibition of both Ca²⁺ influx and Ca²⁺ release from intracellular Ca²⁺ stores.