Comparison of the effects of caffeine and procaine on noradrenergic transmission in the guinea‐pig mesenteric artery

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Abstract
The effects of caffeine and procaine on noradrenergic transmission in the guinea-pig mesenteric artery were investigated by recording electrical responses of smooth muscle cells and by measuring the outflow of noradrenaline [norepinephrine] (NA) and 3,4-dihydroxyphenylglycol (DOPEG) induced by perivascular nerve stimulation. Caffeine possessed dual actions on the membrane, i.e., at low concentrations (2.5 .times. 10-4-5 .times. 10-4 M), it hyperpolarized the membrane and decreased the membrane resistance and at high concentrations (over 2.5 .times. 10-3 M) it depolarized the membrane and increased the membrane resistance. Procaine (over 10-4 M) consistently depolarized the membrane and increased the membrane resistance. The amplitude of the excitatory junction potential (ejp) produced by perivascular nerve stimulation was increased by low concentrations of procaine (2.5 .times. 10-5-10-4 M) or high concentrations (10-3-5 .times. 10-3 M) of caffeine and was decreased by low concentrations of caffeine (2.5 .times. 10-5-10-4 M) or high concentrations of procaine (5 .times. 10-4-10-3 M). Higher concentrations of caffeine (over 5 .times. 10-3 M) induced a spike potential on the ejp, while higher concentrations of procaine (over 2.5 .times. 10-3 M) inhibited the generation of ejp. Facilitation of ejp produced by repetitive stimulation of perivascular nerves remained unchanged by caffeine, while it was enhanced by procaine at any given concentration (caffeine 2.5 .times. 10-4-10-3 M; procaine 10-4-10-3 M). The membrane depolarization produced by exogenously applied NA (10-5 M) was not blocked by pretreatment with procaine. Conduction velocity of perivascular nerve excitation remained unchanged by application of caffeine (up to 5 .times. 10-3 M), and was reduced by application of procaine (over 2.5 .times. 10-4 M). Outflow of NA during perivascular nerve stimulation remained unchanged by caffeine (10-4-3 .times. 10-3 M), while it was enhanced by procaine (over 2.5 .times. 10-4 M). The outflow of DOPEG was slightly reduced by caffeine (10-3-5 .times. 10-3 M) and by lower concentrations of procaine (10-4-2.5 .times. 10-4 M) but was not altered by higher concentrations of procaine (10-3-5 .times. 10-3 M). In the guinea-pig mesenteric artery, high concentrations of caffeine (over 10-3 M) increased the ejp amplitude which might be due to an increase in membrane resistance of the smooth muscle cells. No marked effect of caffeine was observed on transmitter release from the nerve terminals. Procaine (over 2.5 .times. 10-4 M) increased transmitter release from perivascular nerves and blocked the re-uptake mechanism of released NA. The mechanisms underlying the decrease in ejp amplitude by procaine remain to be determined.