The role of calcium and alpha-adrenoceptors in contractile response of chick expansor secundariorum muscle to field stimulation

1. The effects of some alpha-adrenergic agonists and antagonists on electrically-evoked contractions and tension of chick expansor secundariorum muscle (ESM), and dependence of these events on extracellular calcium was investigated. 2. Both train and continuous electrical stimulation can produce regular contractions in preparations obtained from 40-60 day old chicks. 3. Clonidine had a biphasic action on the contractions produced by train electrical stimulation. In concentrations ranging from 10^-8 to 3 × 10^-7 M, clonidine decreased the contraction amplitude, but in higher concentrations, it caused an increase in both the muscle tension and the contraction amplitude. These effects were reversed by application of yohimbine although yohimbine by itself had no effect on the contractions. 4. Introduction of calcium free isotonic high potassium medium decreased muscle tone which was followed by further dose-dependent increase in tension, along with the addition of cumulative doses of CaCl₂ (ED₅₀ = 2.8 × 10^-3 M). 5. Nifedipine reduced the amplitude of ESM contractions produced by continuous electrical stimulation in a dose dependent manner (IC₅₀ = 6.7 × 10^-7 M). 6. Methoxamine induced a completely dose dependent increase in muscle tension which was dependent on extracellular calcium and was inhibited by nifedipine. In the presence of 10^-8 M nifedipine, ED₅₀ of methoxamine stimulatory effect increased from the control value of 2.2 × 10^-7 to 8.4 × 10^-7 M. 7. Although BAY-K8644, a calcium channel activator, had no effect on the contraction amplitude produced by continuous electrical stimulation, the muscle tension was increased by this drug in a dose dependent manner (ED₅₀ = 2.2 × 10^-8 M).