EFFECTS OF MAGNESIUM ON CONTRACTILE RESPONSES INDUCED BY ELECTRICAL TRANSMURAL STIMULATION AND NORADRENALINE IN RABBIT THORACIC AORTA

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Abstract
1 In rabbit isolated thoracic aortae, effects of magnesium ions on the contraction and 3H-efflux in response to electrical transmural stimulation and on the contractile responses induced by noradrenaline and KCl were investigated. 2 Addition of magnesium (1.2, 3.6, 12.0 and 24.0 mm) to the bathing solution inhibited the electrically induced contractions in a dose-related manner; the inhibition was complete with a concentration of 24.0 mm. 3 The increase in 3H-efflux induced by electrical transmural stimulation was inhibited by the addition of magnesium to the superfusing fluid, but a complete block was not obtained even in high concentrations of magnesium. 4 Magnesium ions (1.2, 3.6, 12.0 and 24.0 mm) inhibited the contractile responses induced by low concentrations of noradrenaline (2 × 108m) and KCl (2 × 10 −2m). However, the responses induced by higher concentrations of noradrenaline (5 × 10−7 and 10−5m) and KCl (3 and 4 × 10−2m) were enhanced by low concentrations of magnesium. 5 Magnesium ions affect both presynaptic and effector sites in rabbit thoracic aortae but in a different manner; magnesium manifests only an inhibitory effect on noradrenaline release from the adrenergic nerves, but dual effects on reactivity of vascular smooth muscle, depending on concentrations of magnesium and stimulants; it is suggested that the change in vascular reactivity is more important than the reduction in transmitter release when magnesium inhibits the response to nerve stimulation.