Effects of Phenylephrine on the Contractile Tension and Cytosolic Ca²⁺Level in Rat Anococcygeus Muscle

Abstract

A contractile propertiy of phenylephrine (PE), α₁ agonist, on rat anococcygeus muscle was compared with that on rat aorta by simultaneously measuring changes in intracellular Ca²⁺ ([Ca²⁺] ₁) level and muscle tension.(1) PE (0.1-30, μ M) and high K⁺induced a sus tained increases in [Ca²⁺] ₁ level and muscle tension of both the muscles.(2) An application of verapamil (10μM) and EGTA (4mM) decreased the PE or high K+-increased tension and [Ca2+] 1 level in both the muscle, respectively.(3) A cumulative application of PE or high K⁺to anococcygeus muscle and aorta exhibited a positive relationship between [Ca²⁺] ₁ and developed tension. The developed tension by PE was greater than that by high K⁺at the same level of [Ca²⁺] ₁ only in the aorta. A difference of regression slopes in the relationship between [Ca²⁺] ₁ level and muscle tension under PE and high K⁺-treatments in aorta was significant, but that in anococcygeus muscle was not.(4) An application of PE to anococ cygeus muscle in Ca²⁺free medium elicited a small transient contractile tension and increase in [Ca²⁺] ₁ level, but that to aorta showed a large and transient increase in both the parameters.(5) Phorbol ester, DPB (1μM), did not affect muscle tension or [Ca²⁺] ₁ level in anococcygeus muscle, but DPB induced greater increases in aorta.(6) An application of PE (10μM) with GTP produced a left shift in the pCa-tension curve in the β-escin-permeabilized fiber of the anococcygeus muscle.
In summary, it is suggested that the sustained contraction induced by PE in anococ cygeus muscle is involved with the increases in [Ca²⁺] ₁ which is due to Ca²⁺influx mediated by a, receptor, but scarecely to Ca²⁺release from the intracellular storage, and that an increase in Ca²⁺sensitivity to PE is found only in the permeabilized anococcygeus muscle. The Ca²⁺-independent contractile mechanism in PE response as seen in aorta is probably to be absent in anococcygeus muscle. Moreover, it seems that the effect of the drug acting protein kinase C on anococcygeus muscle is extremely lesser than that on aorta.