Contraction phasic/tonic

Specialization Each receptor contributes to morphologically (e.g., phasic vs. tonic) or temporally (early vs. late) distinct components of the overall response. This is the case of prolonged EFS-induced nonadrenergic, noncholinergic contraction the guinea pig colon or rat urinary bladder, where the fast and late components of the contraction are mediated by NKxror NK2r, respectively... 

The rise in [Ca2+] and MLCK phosphorylation of myosin need to be reversed for relaxation. It is very important to the correct functioning of the uterus that its contractions are phasic and not maintained (tonic), as blood flow falls in the myometrium at the peak of contraction, due to the occlusion of its blood vessels (Larcombe-McDouall et al 1999). Relaxation involves a cessation of voltage-gated... 

Horikawa Y, Goel A, Somlyo AP, Somlyo AV 1998 Mitochondrial calcium in relaxed and tetanized myocardium. Biophys J 74 1579—1590 Horiuti K, Somlyo AV, Goldman YE, Somlyo AP 1989 Kinetics of contraction initiated by flash photolysis of caged adenosine triphosphate in tonic and phasic smooth muscles. J Gen Physiol 94 769-781... 

Incubation in 1-20 iM pyridine for 20 min inhibited norepinephrine-induced phasic and tonic contractions in the thoracic aorta, incubated as aortic rings, as well as the endothelium-denuded aorta of Wistar rats (Hsu Lin-Shiau, 1995). These effects were related to inhibition of the calcium influx normally elicited by norepinephrine. 

Nakaki T, Roth BL, Chuang DM, Costa E. Phasic and tonic components in 5-HT2 receptor-mediated rat aorta contraction participation of Ca++ channels and phospholipase C. J Pharmacol Exp Ther 1985 234 442-446. 

The leucokinins (LK s) are a new class of insect myotropic neuropeptides isolated from head extracts of the cockroach L. maderae. These octapeptides all contain a similar core sequence of 5 amino acids that extend from position 4 through 8. This sequence Phe-X-Ser-Trp-Gly-NH2 seems to be required for hindgut stimulation. The initial response of the hindgut to the LK s was characterized by an increase in the frequency and/or amplitude of phasic contractions (50, ). At higher peptide concentrations, a tonic component was generally present. All of the LK s showed a response at 3 x 10 M that was 5-10% above the mean level of spontaneous activity. The maximum response for each of the peptides was recorded at a concentration 2.1 X 10" M. Thus, the intrinsic activities for the LK s are nearly equal because the dose-response curves have about the same asymptotic limits. A comparison of the dose concentrations that gave a half maximal response (ED q) for the 8 peptides is shown in Table I. 

Smooth muscle exhibits very diverse behaviors depending on which control mechanisms are present. Vascular smooth muscle, for example, lacks fast voltage-dependent Na+ or Ca + channels and so does not have action potentials or Ca + spikes. It has slow voltage-dependent Ca + channels that admit calcium in a graded fashion in response to fluctuations in membrane potential induced by humoral or transmitter effects on membrane ion conductances, and it has several membrane receptor-initiated second-messenger cascades that control Ca " " entry and Ca + release from its limited SR, and which moderate the effectiveness of Ca +. Vascular smooth muscle contraction is thus tonic rather than phasic, and is very dependent on extracellular Ca + therefore Ca + channel blockers effectively inhibit contraction. In contrast, gut smooth muscle does have fast voltage-dependent channels sufficient to produce action potentials and more SR than vascular smooth muscle, and also has gap junctions through which ion fluxes can occur. It also has receptor-mediated Ca +... 

Hurwitz et al. (32) extended these ideas by giving evidence, again in a nonsecretory tissue, for two potential dependent calcium channels. These authors showed that the calcium channel associated with the phasic contraction of guinea pig ileal smooth muscle was blocked by lanthanum, but the calcium channel mediating the tonic contraction was not. In this system, both these channels were potential dependent. Hence, a variety of calcium channels may exist on cell surfaces. 

Because earlier studies in this and other laboratories showed that inhibitors of tyrosine kinase activity suppressed contraction of smooth muscle (Section II), we suspected that vanadate-induced contraction might be due to its efficacy as an inhibitor of tyrosine phosphatase activity. Our experiments showed that treatment of taenia coli with 1.5 mM vanadate induced a tonic contraction that was associated with enhanced tyrosine phosphorylation of at least three substrates (M 85,000, 116,000, and 205,000 Figs. 2A and 2B). Maximal force attained in response to vanadate was about one-fourth to one-third of the maximal force attained during the phasic contraction elicited by stimu-... 

Smooth muscles can be divided into two main groups phasic and tonic muscle types. Phasic muscle, e.g. taenia coli, exhibits spontaneous action potentials and have faster contractile kinetics whereas tonic muscle, e.g. the aorta, do not have spontaneous activity and contract more slowly (cf. Somlyo and Somlyo 1968, Horiuti et al. 1989). Comparative studies have revealed that the shortening velocity of smooth muscles span of over a wide range (Malmqvist and Arner 1991). A fast smooth muscle, e.g. rabbit rectococcygeus, can have a shortening velocity that is about 7-fold faster than that of a slow smooth muscle, e.g. the aorta. This difference in velocity is similar to that between fast and slow skeletal muscle fibres. Thus smooth muscle is a heterogeneous group of muscles with a span of different kinetic properties in their contractile systems. 

Wingard CJ, Paul RJ, Murphy RA (1997) Energetic cost of activation processes during contraction of swine arterial smooth muscle. J Physiol (Lond) 501 213-223 Word RA, Tang DC, Kamm KE (1994) Activation properties of myosin light chain kinase during contraction/relaxation cycles of tonic and phasic smooth muscles. J Biol Chem 269 21596-21602... 

Fig. 4. Phenylephrine-induced contraction in intact portal vein smooth muscle showing the effect of 48 hr treatment with DC3B, the chimeric toxin that ADP-ribosylates and inhibits the activity of endogenous RhoA. Note that the contractile response of untreated smooth muscle was biphasic, consisting of a phasic transient followed by a tonic phase. DC3B treatment inhibited the tonic phase of contraction with little effect on the initial, transient phase (P< 0.0001 from Fujihara et al. 1997).

Male reproductive system. Lanthanum abolished both the tonic and the phasic KCl-induced contractions in the rat vas deferens, and this was not reversed by Ca (Hay and Wadsworth 1982). Lanthanum reduced the Ca -dependent contractions induced by oxytocin in the isolated testieular capsule of the rat (Sanchez, Manso et al. 1989). 

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