Mechanisms of caffeine‐induced contraction and relaxation of rat aortic smooth muscle.

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Abstract
1. Using front‐surface fluorimetry and Fura‐2, we determined the effects of caffeine on cytosolic calcium concentration ([Ca2+]i) and on tension of strips of the rat thoracic aorta. We also determined the effects of caffeine on 45Ca2+ influx into the strips. The objective was to elucidate the mechanism of contraction and relaxation in vascular smooth muscle, as induced by caffeine. 2. In normal physiological salt solution (PSS), caffeine induced a transient tension development, while it induced a biphasic change in [Ca2+]i. The initial transient peak in [Ca2+]i which coincided with tension development was followed by a sustained increase. Thus, changes in tension did not follow changes in [Ca2+]i. In Ca(2+)‐free PSS, both the caffeine‐induced contraction and the increase in [Ca2+]i were transient. It was suggested that in both the presence and absence of extracellular Ca2+, the transient increase in [Ca2+]i was due to the release of Ca2+ from the intracellular store. Although the sustained increase in [Ca2+]i depended on extracellular Ca2+, it was not affected by diltiazem, a Ca2+ antagonist. 3. Caffeine inhibited the increase in [Ca2+]i and tension development during 118 mM‐K+ depolarization, in a concentration‐dependent manner. The extent of reduction in tension (relaxation) was greater than that expected from the reduction in [Ca2+]i based on the [Ca2+]i‐tension relationship observed with K+ depolarization. Pretreatment of the strips with ryanodine did not alter the inhibitory effects of caffeine. 4. Caffeine inhibited the increased [Ca2+]i and developed tension during stimulation by 10(‐5) M‐noradrenaline, in a concentration‐dependent manner. 5. Dibutyryl cAMP (10(‐4) M) inhibited both high K(+)‐induced and noradrenaline‐induced tension development. Inhibition of an increase in [Ca2+]i in relation to the inhibition of tension during noradrenaline stimulation was much greater than that in 118 mM‐K+ depolarization. 6. Although caffeine per se had no effect on 45Ca2+ influx in the strips in normal PSS, caffeine did inhibit the increase in 45Ca2+ influx stimulated by 118 mM‐K+ or by 10(‐5) M‐noradrenaline, to a similar extent and with similar IC50 values. 7. The characteristic features of the effects of caffeine on vascular smooth muscle, i.e. the transient nature of contraction and the relaxation of precontracted strips could be explained as follows: caffeine is able to reduce [Ca2+]i after releasing Ca2+ from intracellular stores; however, this may play a minor role. Independent of the [Ca2+]i reduction, the second messenger, cAMP, might directly influence the [Ca2+]i‐tension relationship, and if so, would play a major role.