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The effects of isoprenaline (10(-6) M) on relaxation, unidirectional as well as net Ca(2+) fluxes, and cyclic AMP levels were investigated in rabbit aorta under the condition of high-K(+) depolarization in the presence of phentolamine (10(-5) M).2. Isoprenaline (10(-6) M) caused significant inhibition of Ca(2+) influx stimulated by 145 mM-K(+) (0 Na(+)) solution. The time courses of Ca(2+) influx inhibition and relaxation by isoprenaline were parallel. Isoprenaline also caused a significant inhibition of high-K(+)-induced gain in net Ca(2+) content.3. Ro 20-1724 (1 mM), a phosphodiesterase inhibitor, also caused relaxation and Ca(2+) influx inhibition in high-K(+)-depolarized rabbit aorta. Pre-treatment with Ro 20-1724 potentiated isoprenaline-induced Ca(2+) influx inhibition and relaxation.4. Isoprenaline and Ro 20-1724 each alone increased cyclic AMP levels. Furthermore pre-treatment with Ro 20-1724 caused potentiation of isoprenaline-induced increases in cyclic AMP levels.5. At submaximal concentration, D600 (10(-7) M) caused partial inhibition of high-K(+)-stimulated Ca(2+) influx and produced relaxation. However, unlike Ro 20-1724, it did not potentiate isoprenaline-induced Ca(2+) influx inhibition and relaxation. D600 does not increase cyclic AMP levels in smooth muscle.6. Dibutyryl cyclic AMP (1 mM), a lipid-soluble analogue of cyclic AMP, caused relaxation and inhibited high-K(+)-stimulated Ca(2+) influx.7. Isoprenaline failed to cause stimulation of Ca(2+) efflux in high-K(+)-depolarized rabbit aorta.8. It is concluded that the inhibition of Ca(2+) influx may be one of the mechanisms by which beta-receptor stimulation can reduce intracellular free Ca(2+) to promote relaxation of smooth muscle. The data support the involvement of cyclic AMP in this action of the beta-agonist.9. Since the experiments were conducted in 145 mM-K(+) (0 Na(+)) depolarizing conditions, the role of hyperpolarization or of a Na(+)-Ca(2+) exchange mechanism in isoprenaline-induced Ca(2+) influx inhibition and/or relaxation can be excluded.
Meisher et al. (1982) studied this question.