EFFECTS OF PROCAINE AND EXTRACELLULAR CALCIUM CONCENTRATION ON RESPONSE OF RAT STOMACH FUNDUS MUSCLE TO ACETYLCHOLINE AND 5‐HYDROXYTRYPTAMINE

When rat stomach fundus muscle was incubated for 30 min in Tyrode solution from which calcium chloride had been omitted, there was an almost complete abolition of the contractile response to 5‐hydroxytryptamine (5‐HT) while that to acetylcholine (ACh) was still present. The maximum tension obtainable with ACh remained the same in external calcium concentrations ranging from 0.45 to 3.6 mm, but the pD₂ value increased. A concentration of at least 0.9 mm calcium was needed to maintain a maximum contraction with 5‐HT, and the pD₂ for this agent also increased significantly with increase in calcium content of the medium. The effects of procaine on the responses of the muscle to 5‐HT and ACh were similar to the respective changes induced by lowering the calcium concentration, and were reduced by the addition of calcium. Concentrations of 2.2 × 10⁻⁷ to 3.6 × 10⁻⁵m procaine reduced the effects of both 5‐HT and KCl and suppressed the maximum responses. The maximum responses to KC1 and 5‐HT were restored at higher concentrations of procaine (>3.6 × 10⁻⁴m), while the effect of ACh was reduced. It is suggested that 5‐HT, like KC1, is almost entirely dependent on extracellular calcium for inducing muscle contraction, while ACh may utilize calcium from bound stores. 1979 British Pharmacological Society