Muscle tone and drug sensitivity of isolated bovine iris sphincter are enhanced at low temperatures

The effects of temperature on nerve-mediated or carbachol-induced contractions of isolated bovine iris-sphincter muscles were studied. A decrease in temperature (to below 36 degrees C) led to contraction of the iris sphincter. The contractile responses induced by intramural cholinergic nerve stimulation were augmented by moderate cooling (29 degrees-31 degrees C), but were markedly inhibited by severe cooling (less than 22 degrees C). On the other hand, after cooling the contractile amplitude induced by carbachol (3 X 10(-7) M) was enhanced to 258% +/- 16% (29 degrees-31 degrees C), and 185% +/- 12% (20 degrees-21 degrees C) of control levels. Whereas the cooling-induced development of muscle tone was inhibited by the presence of indomethacin and FPL-55712, the application of prostaglandins (10(-9) to 3 X 10(-8) M) led to restoration of the sensitivity of the muscle observed at low temperatures. As low temperatures increase the response of bovine iris-sphincter muscle, temperature might affect the state of the iris during intraocular clinical surgery and in cases of intraocular inflammation.