Phosphorylation of myosin light chain and phosphorylase in tracheal smooth muscle in response to KCl and carbachol.
It has been proposed that Ca2+-dependent myosin light chain (P-light chain) phosphorylation in smooth muscle permits cycling of myosin cross-bridges within myofibrillar elements for muscle shortening, but a second Ca2+-dependent regulatory mechanism is responsible for force generation. Accordingly, we examined P-light chain phosphorylation and another Ca2+-dependent protein phosphorylation reaction, phosphorylase a formation, in bovine tracheal smooth muscle during isometric force generation elicited by the cholinergic agonist carbachol or KCl depolarization, two stimuli thought to increase the concentration of sarcoplasmic free Ca2+ by mobilizing different pools of Ca2+. Increases in P-light chain phosphorylation reached maximal values of 0.79 and 0.59 mole of phosphate per mole of P-light chain at 1 min and then declined during maintained isometric force developed in response to 1 microM carbachol and 60 mM KCl, respectively. Carbachol elicited approximately twice the amount of force as found in the presence of KCl, and yet a more rapid rate of decline in the phosphate content of P-light chain was apparent. Decreases in maximal levels of phosphorylase a also occurred during carbachol-mediated isometric force maintenance, yet did not occur with KCl stimulation. Concentration-dependent responses with carbachol and KCl showed a positive relationship between the extent of P-light chain phosphorylation and extent of developed isometric force after 1 min of contraction with both stimuli. Under no conditions was force generated without P-light chain phosphorylation. The concentration dependence of phosphorylase a formation with KCl was similar to isometric force and P-light chain phosphorylation. However, concentrations of carbachol necessary to stimulate phosphorylase a formation were much higher than those required for stimulation of isometric force and P-light chain phosphorylation. Furthermore, carbachol attenuated the stimulation of phosphorylase a formation by isoproterenol. Thus, carbachol appears to have both an inhibitory and stimulatory effect on phosphorylase a formation in bovine tracheal smooth muscle. These results also indicate that maintained isometric force in smooth muscle may be dependent upon the maximal extent of P-light chain phosphorylation obtained during an early temporal transient in phosphorylation.
D000818:Animals; D002118:Calcium / Q000502:physiology; D002217:Carbachol / Q000494:pharmacology; D002417:Cattle; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D010762:Phosphorylase a / Q000096:biosynthesis; D010766:Phosphorylation; D011189:Potassium Chloride / Q000494:pharmacology; D011976:Receptors, Muscarinic / Q000502:physiology; D014132:Trachea / Q000378:metabolism
D011976:Receptors, Muscarinic; D011189:Potassium Chloride; D002217:Carbachol; D010762:Phosphorylase a; D009218:Myosins; D007545:Isoproterenol; D002118:Calcium