Endogenous Prostaglandins and Spontaneous Contractions in the Circular Muscle of the Guinea-Pig Stomach

Abstract

The physiological significance of endogenous prostaglandin (PG) on spontaneous contractions in isolated circular muscles of the guinea-pig stomach was investigated.Contents of the E and F types of PG were measured by radioimmunoassay as PGE₂ and PGF_2α.The isolated circular muscles contained 17.04±2.12ng PGE₂/g tissue (n=12) and 9.38±0.62ng PGF_2α/g tissue (n=12), and released 0.36±0.04ng PGE₂/g tissue/min (n=10) and 0.14±0.01 ng PGF_2α/g tissue/min (n=12) into the solution. These contents and releases of PGs were reduced by treatment with indomethacin (ID₅₀=0.3μEg/ml).Indomethacin (0.1-6μEg/ml) increased the amplitude of spontaneous contractions dosedependently, while high concentrations (20μEg/ml) conversely depressed it.Physiological concentrations of exogenously applied PGE₂ (0.03-0.35ng/ml) markedly suppressed spontaneous contractions in the presence and absence of indomethacin (3μEg/ml), while PGF_2α in concentrations of 0.35-3.54ng/ml slightly increased them. Therefore, these results suggested that spontaneous contractions in circular muscles were largely influenced by the E type of endogenous PG, not by the F type. The indomethacin-induced increase in the amplitude of spontaneous contractions was not affected by intrinsic nerves and was accompanied by an increase in spike discharges on the slow wave and ⁴⁵Ca uptake in the muscle cells.These observations suggested that the E type of endogenous PG in circular muscles might play a suppressive role in spontaneous contractions by inhibiting spike generation on the slow wave.