Abstract
1 The relation of calcium ion influx into nerve terminals to presynaptic adrenoceptor function and the possible masking, by desensitization due to intraneuronal calcium accumulation, of the effects of adrenoceptor agonists and antagonists on presynaptic alpha-adrenoceptors was investigated in guinea-pig atria previously incubated with [(3)H]-noradrenaline.2 Atria were stimulated with 100 pulses at various frequencies (1 to 15 Hz) in standard (2.3 mm), low (0.26 mm) and high (6.9 mm) calcium-Krebs solution in the absence and then the presence first of noradrenaline and subsequently phenoxybenzamine.3 The per pulse overflow of tritium was directly related to the calcium concentration of the Krebs solution, being much reduced and substantially increased in 0.26 and 6.9 mm calcium-Krebs solutions respectively.4 Noradrenaline inhibited the overflow of tritium in low calcium-Krebs solution, to a relatively constant extent, independently of frequency. In addition, the agonist had a greater maximal inhibitory effect in standard than in reduced calcium-Krebs. The catecholamine was as effective an inhibitor of overflow at the lowest and highest frequencies in high as it was in standard calcium-Krebs solution. Phenoxybenzamine invariably ...Continue Reading
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