Functional importance of the actin cytoskeleton in contraction of bovine iris sphincter muscle

1. The contractile capacity of smooth muscle cells depends on the cytoskeletal framework of the cell. The aim of this study was to determine the functional importance of both the actin and the tubulin components of the cytoskeleton in contractile responses of the bovine isolated iris sphincter muscle. 2. In each preparation, two contractions to the muscarinic agonist carbachol were obtained. The maximum responses of the first contractions were taken as 100%. The second contractions to carbachol were elicited in the presence of either cytochalasin B (50 and 5 microm), an inhibitor of the actin cytoskeleton, or colchicine (100 microm), an inhibitor of the tubulin cytoskeleton (30 min incubation). 3. Cytochalasin B, at a concentration of 50 microm, significantly decreased the contractions induced by carbachol, with the maximum response reduced to 21.8 +/- 6.6% (n = 12) of the initial maximum. The maximal contractions to carbachol in the presence of colchicine reached 96.2 +/- 7.9% (n = 9) of the initial contraction, which was not significantly different from control second responses to carbachol with neither drug present, which reached 113.3 +/- 7.6% (n = 7). 4. The effect of cytochalasin B was dose-dependent, since at a lower concentration of 5 microm, the drug decreased the maximum contraction to carbachol to 60.3 +/- 8.8% (n = 6). The effect of cytochalasin B was at least partially reversible, since after the use of the higher concentration of 50 microm, contractions to carbachol increased to 62.3 +/- 15.5% (n = 4) of the maximal response, after 1 h repeated washing of the preparations. 5. Cytochalasin D, at a concentration of 50 microm, completely abolished the contractions induced by carbachol (n = 4). 6. These findings suggest that in bovine iris sphincter muscle, contractions to carbachol are highly dependent, from a functional point of view, on actin polymerization, and not, to any important degree, on the polymerization of tubulin.