Effects of microtubule disruption on force, velocity, stiffness and [Ca2+](i) in porcine coronary arteries

Force generated by smooth muscle cells is believed to result from the inter action of actin and myosin filaments and is regulated through phosphorylati on of the myosin regulatory light chain (LC20). The role of other cytoskele ton filaments, such as microtubules and intermediate filaments, in determin ing the mechanical output of smooth muscle is unclear. In cultured fibrobla sts, microtubule disruption results in large increases in force similar to contractions associated with LC20 phosphorylation (15). One hypothesis, the "tensegrity" or "push-pull" model, attributes this increase in force to th e disruption of microtubules functioning as rigid struts to resist force ge nerated by actin-myosin interaction (9). In porcine coronary arteries, the disruption of microtubules by nocodazole (11 muM) also elicited moderate bu t significant increases in isometric force (10-40% of a KCl contracture), w hich could be blocked or reversed by taxol (a microtubule stabilizer). We t ested whether this nocodazole-induced force was accompanied by changes in c oronary artery stiffness or unloaded shortening velocity, parameters likely to be highly sensitive to microtubule resistance elements. Few changes wer e seen, ruling out push-pull mechanisms for the increase in force by nocoda zole. In contrast, the intracellular calcium concentration, measured by fur a 2 in the intact artery, was increased by nocodazole in parallel with forc e, and this was inhibited and/or reversed by taxol. Our results indicate th at microtubules do not significantly contribute to vascular smooth muscle m echanical characteristics but, importantly, may play a role in modulation o f Ca2+ signal transduction.