CAFFEINE ACTING ON PREGNANT RAT MYOMETRIUM - ANALYSIS OF ITS RELAXANT ACTION AND ITS FAILURE TO RELEASE CA-2+ FROM INTRACELLULAR STORES

1. The effect of caffeine on mechanical activity was studied in pregnant rat myometrium. 2. In muscle cells with intact plasmalemmae, caffeine (0.1-50 mM) produced no contraction whatever the experimental conditions. 3. Caffeine (0.1-10 mM) inhibited, in a concentration-dependent manner, contractions induced by electrical stimulation, potassium-rich (60 mM K+) solution, sodium-free solution or oxytocin (22.5 nM). 4. In Ca2+-free solution, various substances (oxytocin, sodium orthovanadate and prostaglandin E2) evoked sustained contractions that were suppressed by caffeine (5-10 mM). When caffeine (>5 mM) was applied during Ca2+-loading of the tissue (2.1 mM Ca2+, 5 min) in the presence of a K+-rich solution, the subsequent transient contraction induced by a short application (10 s) of oxytocin (22.5 nM) in Ca-free solution was reduced (63±3.5% reduction for 20 mM caffeine, n = 4). 5. In saponin-skinned strips, application of caffeine (5-10 mM) during loading of the Ca2+-store increased the subsequent contraction induced by myo-inositol 1,4,5 trisphosphate (IP3, 10 μM). Caffeine (10-30 mM) decreased calcium-activated contractions in skinned fibres lacking a functional internal Ca-store. This effect was reduced by the cyclic AMP-dependent protein kinase inhibitor Thr-Thr-Tyr-Ala-Asp-Phe-Ile-Ala-Ser-Gly-Arg-Thr-Gly-Arg-Arg-Asn-Ala-Ile- His-Asp (8 μM). 6. In conclusion, it is suggested that the inability of caffeine to cause spasm of rat myometrium is due to the absence of caffeine-sensitive calcium-release channel in the sarcoplasmic reticulum. Relaxant effects of caffeine can be explained be mechanisms leading to a decrease in both the cytoplasmic free Ca2+ concentration and the Ca2+-sensitivity of the contractile machinery.