Calcium requirement for the α-action of catecholamines on guinea-pig taenia coli

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Abstract
The effect of removal and readmission of Ca2+ on the response of guinea pig taenia coli to the .alpha.-action of adrenaline [epinephrine] and noradrenaline [norepinephrine], i.e., hyperpolarization and decrease in membrane resistance, was studied with the double sucrose-gap method. Depolarization and reduction of membrane resistance caused by Ca removal were prevented by raising the external Mg concentration since high Mg2+ up to complete substitution of the external Na+ with Mg2+ (93 mM Mg), did not significantly modify the effect of adrenaline in the presence of the normal concentration of Ca2+ (2.5 mM). In Ca-free solution containing 124 mM Na, 12 mM Mg and 0.5 mM EGTA, the adrenaline effect gradually disappeared within 10-20 min. The recovery of the adrenaline effect was usually complete within 5 min after readmission of 2.5 mM Ca. When external Na+ was low or absent and Mg2+ was high it was more difficult to abolish the response to adrenaline by removal of Ca. Restoration of the normal external Ca concentration in the presence of low Na and high Mg produced slow and incomplete recovery of the response to adrenaline. A brief transient application of Ca (5 mM for 15-30 s) to a muscle superfused with Ca-free solution produced hyperpolarization and increase in membrane resistance. A similar Ca application in Ca-free solution containing adrenaline caused hyperpolarization with a reduction of membrane resistance. The efficacy of Ca causing hyperpolarization associated with a reductin of membrane resistance in the presence of adrenaline was stronger when the external Na concentration was high and the Mg concentration was low than in the presence of high Mg and low or 0 Na. The presence of Ca is apparently essential for the hyperpolarization and increase in membrane conductance caused by adrenaline. The site of Ca involvement is probably at the inner surface of the membrane. Mg2+ and Na2+ may affect response to adrenaline through modification of the amount of Ca at the strategic site by influencing Ca movement across the membrane.

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