Different effects of substance P and vasoactive intestinal peptide on the motor function of bovine intraocular muscles.

Effects of substance P (SP) and vasoactive intestinal peptide (VIP) on motor function of bovine sphincter, dilator, and ciliary muscles were investigated pharmacologically, with and without field stimulation. SP contracted the sphincter and the maximum response was only about 25% of that in the presence of equimolar doses of carbachol. SP had no significant influence on the bovine dilator, and VIP lowered the tone of the iris muscles. The responses to these peptides seemed mainly to be caused by a direct action on the smooth muscles of the iris. The ciliary muscle did not respond to application of these peptides. There was no influence on the amplitude of the acetylcholine- or norepinephrine-induced responses of any of these bovine intraocular muscles. The nerve-mediated responses of the iris muscles were not altered by application of low doses of these peptides, yet with doses over 5 X 10(-6) M, the evoked response of the iris muscles was lowered. In the ciliary muscle tissue, high concentrations of SP slightly decreased the evoked contraction. These observations indicate that SP and VIP play different roles on the motor function of the bovine intraocular muscles.