α₁-Adrenoceptor-mediated negative inotropic effect caused by intracellular ionic activities in guinea-pig papillary muscle

In guinea-pig papillary muscle, phenylephrine (PE), an agonist of α₁-adrenoceptor (α₁-AR), led to a transient negative inotropic effect (NIE) and a subsequent sustained positive inotropic effect (PIE). To clarify the ionic mechanisms underlying the NIE, we measured the [Na⁺]i or [pH]i by ion-selective microelectrodes. PE produced a decrease in the intracellular Na⁺ concentration ([Na⁺]i) and an increase in intracellular pH ([pH]i). During the phase of NIE, PE produced only a (-) change of [Na⁺]i (Δ[Na⁺]i). With a decrease in extracellular Na⁺ or an increase in extracellular Ca²⁺, the PE-induced NIE was attenuated and PE produced (+)Δ[Na⁺]i. The PE-induced NIE and (-)Δ[Na^+z]i were definitely strengthened by lowering the bath temperature or increasing the stimulation frequency. 2-(2,6-di-methoxyphenoxyethyl)amino-methyl-1,4- benzidioxane HC1, an antagonist of α_1A-AR, completely abolished the PE-induced NIE and (-)Δ[Na⁺]i. Phorbol 12,13-dibutyrate, an activator of protein kinase C (PKC), decreased the baseline [Na⁺]i and twitch force and increased the baseline [pH]i in mimicry of PE. Pretreatment with 1-5(isoquinoline-sulphonyl)-2-methylpiperazine, an inhibitor of PKC, abolished the PE-induced NIE and (-)Δ[Na⁺]i. During pretreatment with benzamil, an inhibitor of Na⁺/Ca²⁺ exchange, we found that the PE-induced NIE and (-)Δ[Na⁺]i were reversibly abolished. Our results indicate the PE-induced NIE may be elicited upon the activation of Na⁺/Ca²⁺ exchange which can be attributed to the (-)Δ[Na⁺]i. (-)Δ[Na⁺]i is mediated through a PKC-dependent pathway via an activation of α_1A-AR subtype and its effect could be strengthened remarkably at high [Na⁺]i and [Ca²⁺]i values.