Effects of indomethacin and prostaglandins on the dog iris sphincter and dilator muscles.

Effects of indomethacin and prostaglandins (PGs: PG E1, PG E2 and PG F2 alpha) on electrical and mechanical properties of the dog iris sphincter and dilator muscles were studied using isometric tension recording and microelectrode methods. When field stimulations (50 V, 0.4 msec, 10 stimuli at 10 Hz) were applied every 1 min, the muscle tone was gradually elevated in both muscle tissues. Application of 10(-6) M indomethacin gradually reduced the tone of the muscle tissues; however, this reduction was more pronounced in the sphincter muscle. On the contrary, exogenously applied PG F2 alpha increased the muscle tone in both tissues. Ca-free solution reduced the tone in both muscle tissues. Following pre-treatment with indomethacin, Ca-free solution had no effect on the muscle tone in the sphincter; however, it did reduce the tone in the dilator muscle. Indomethacin (10(-6) M) or PGs (PG E1, PG E2 and PG F2 alpha: up to 10(-7) M) had no effect on the resting membrane potential of both muscles. PG F2 alpha dose-dependently contracted both of the sphincter and dilator muscles. PG E1 and PG E2 had little effect on the mechanical properties of these muscles. Neither atropine nor guanethidine affected the amplitude of contractions evoked by PG F2 alpha. These results suggest that endogenous PG F2 alpha may contribute to maintenance of the muscle tone of the sphincter muscle but contributes to a lesser degree in the dilator muscle in the dog iris. PGs apparently act directly on these muscles rather than through the release of adrenergic or cholinergic neurotransmitters.