Polylysine activates smooth muscle actin-myosin interaction without LC20 phosphorylation

Polyamines decrease Ca(2+) sensitivity of tension and increase rates of activation in skinned cardiac myocytes

Owing in part to their interactions with membrane proteins, polyamines (e.g., spermine, spermidine, and putrescine) have been identified as potential modulators of membrane excitability and Ca(2+) homeostasis in cardiac myocytes. To investigate whether polyamines also affect cardiac myofilament proteins, we assessed the effects of polyamines on contractility using rat myocytes and trabeculae that had been permeabilized with Triton X-100. Spermine, spermidine, and putrescine reversibly increased the [Ca(2+)] required for half-maximal tension (i.e., right-shifted tension pCa curves), with the following order of efficacy: spermine (+4) > spermidine (+3) > putrescine (+2). However, synthetic analogs that differed from spermine in charge distribution were not as effective as spermine in altering isometric tension. None of the polyamines had a significant effect on maximal tension, except at high concentrations. After flash photolysis of DM-Nitrophen (a caged Ca(2+) chelator), spermine accelerated the rate of tension development at low and intermediate but not high [Ca(2+)]. These results indicate that polyamines, especially spermine, interact with myofilament proteins to reduce apparent Ca(2+) binding affinity and speed cross-bridge cycling kinetics at submaximal [Ca(2+)].

Regulation of vascular smooth muscle tone by N-terminal region of caldesmon. Possible role of tethering actin to myosin

To assess the functional significance of tethering actin to myosin by caldesmon in the regulation of smooth muscle contraction, we investigated the effects of synthetic peptides, containing the myosin-binding sequences in the N-terminal region of caldesmon, on force directly recorded from single permeabilized smooth muscle cells of ferret portal vein. Two peptides were used, IK29C and MY27C, containing residues from Ile(25) to Lys(53) and from Met(1) to Tyr(27) of the human and chicken caldesmon sequence, respectively, plus an added cysteine at the C terminus. In cells clamped at pCa 6. 7, both peptides increased basal tone. Pretreatment of cells at pCa 6.7 with IK29C or MY27C decreased the amplitude of subsequent phenylephrine-induced contractions but not microcystin-racemic mixture-induced contractions. In all cases the effects of the peptides were concentration-dependent, and IK29C was more potent than MY27C, in agreement with their relative affinity toward myosin. The peptides were ineffective after the phenylephrine contraction was established. MY27C did not further increase the magnitude of contraction caused by a maximally effective concentration of IK29C, consistent with the two peptides having the same mechanism of action. Neither polylysine nor two control peptides containing scrambled sequences of IK29C, which do not bind myosin, had any effect on basal or phenylephrine-induced force. Our results suggest that IK29C and MY27C induce contraction by competing with the myosin-binding domain of endogenous caldesmon. Digital imaging of fluoroisothiocyanate-tagged IK29C confirmed the association of the peptide with intracellular filamentous structures. The results are consistent with a model whereby tethering of actin to myosin by caldesmon may play a role in regulating vascular tone by positioning the C-terminal domain of caldesmon so that it is capable of blocking the actomyosin interaction.

Contractile effects of polycations in permeabilized smooth muscle

The polycations spermine, neomycin and polylysine potentiated Ca(2+)-activated force in beta-escin permeabilized guinea-pig ileum strips. The effect was inhibited by the calmodulin antagonists trifluoperazine, mastoparan and W13. Potentiation was slow or absent in chi-toxin permeabilized strips, indicating dependence on penetration of the polycations into cells. The effects of spermine and neomycin were maintained after extensive permeabilization by beta-escin, which eliminated the contractile effect of GTPgammaS. Replacement of ATP by CTP, which is not a substrate for myosin light chain kinase, inhibited contractile potentiation. Potentiation of Ca(2+)-activated contractions was associated with increased phosphorylation of the myosin regulatory light chains (LC20). A contractile effect of polylysine and neomycin was also seen in Ca(2+)-free medium and after partial LC20 thiophosphorylation, indicating that phosphorylation-independent processes may contribute to the response. Although spermine does not cause contraction in Ca(2+)-free medium at physiological [MgATP], it did so when [MgATP] was lowered to 40 micron. Similar to high-[Mg2+], the rate of contraction on addition of ATP to strips incubated with microcystin-LR in inhibit phosphatase activity was increased by the polycations, but only at [Ca2+] < 0.3 micron. The results suggest that polycations increase Ca(2+)-activated force by inhibiting myosin phosphatase activity, thereby increasing myosin LC20 phosphorylation. However, additional activation mechanisms, evident at low [Ca2+] and at low [ATP] and possibly involving direct activation of myosin, contribute to their effect.

Propiverine hydrochloride, an anti-pollakiuric agent, inhibits the activity of actomyosin ATPase from the urinary bladder

The present study was performed to investigate the effects of propiverine hydrochloride (1-methyl-4-piperidyl diphenylpropoxyacetate hydrochloride, P-4), a novel anti-pollakiuric agent, on the contractile proteins of smooth muscle. P-4 (30-300 microM) inhibited the activity of native actomyosin adenosine triphosphatase (ATPase) that had been freshly purified from canine urinary bladder, and calmodulin at 10 microM overcame this inhibition. P-4 also inhibited myosin light chain kinase from smooth muscle in a dose-dependent manner. However, at 300 microM, P-4 was unable to inhibit by 50% the activity of trypsin-treated myosin light chain kinase, which was independent of Ca2+/calmodulin. 1 mol of calmodulin bound 4 to 5 mol of [14C]P-4 in a Ca2+-dependent manner with a K(d) of 77.4 microM. These results indicate that calmodulin is one of the intracellular target molecules for P-4 and that inhibition of the action of calmodulin by P-4 might cause the inhibition of actomyosin ATPase activity, with subsequent relaxation of the smooth muscle of the urinary bladder.