THE EFFECT OF M AND B 22948 ON CARBACHOL-INDUCED INOSITOL TRISPHOSPHATE ACCUMULATION AND CONTRACTION IN IRIS SPHINCTER SMOOTH-MUSCLE

The effect of a cyclic GMP phosphodiesterase inhibitor, M & B 22948, on carbachol-induced phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown and phosphatidic acid labeling, 1,4,5-inositol trisphosphate (IP3) accumulation and muscle contraction was studied in bovine iris sphincter smooth muscle. Addition of carbachol (10-mu-M) to P-32-labeled tissue resulted in increased labeling of phosphatidic acid and hydrolysis of PIP2. In myo[H-3]inositol labeled tissue, carbachol caused rapid accumulation of IP3 which reached its maximum at about 2 min. Under identical experimental conditions, carbachol initiated a rapid increase in muscle contraction (phasic component) which was followed by a slightly lower contractile response (tonic component) that lasted for several minutes. Pretreatment of the iris sphincter with M & B 22948 did not alter carbachol-stimulated PIP2 breakdown and phosphatidic acid labeling, IP3 accumulation, or phasic component of the contractile response. However, the tonic component of the contractile response was increasingly attenuated by increasing concentrations of the drug. In conclusion, the data presented demonstrate a close correlation between carbachol-induced IP3 accumulation and muscle contraction, and that M & B 22948 does not inhibit carbachol-induced responses in the iris sphincter.