Fast Phasic Contraction Research Articles

Spatiotemporal mapping of the motility of the isolated chicken caecum

We studied the caecal contractile activity of the chicken (Gallus gallus) using single caeca that had been cannulated at their proximal and distal ends, and in paired caeca, maintained in situ on excised segments of gut that were cannulated at the colonic and small intestinal ends. Longitudinal and circular contractile patterns were characterised using high-definition spatiotemporal mapping. Low amplitude longitudinal contraction waves of frequency 14.1 cycles/min occurred in the absence of major contractile events. These were termed fast phasic and appeared to be mediated by slow waves. The nature of major spontaneous contractions occurring in the single caecum varied with the level of caecal distension. Type A contractions occurred when the caecum was not distended, originated from variable sites and propagated in both directions. Type B or C contractile events occurred when the caecum was moderately or fully distended, originated from a predominantly distal site and propagated proximally. On diameter maps, each type B event comprised a succession of contractions which had similar propagation speeds, frequency and direction to fast phasic contractions. Type C events were comprised of a succession of higher amplitude contractions with no appreciable propagation. Perfusion of saline via the colon resulted in fluid entering both caeca and the onset of aborad contractions in their proximal canals. Saline was also seen to flow between caeca during contractile events however no saline was seen to enter the small intestine as has been postulated by other workers.

High-definition spatiotemporal mapping of contractile activity in the isolated proximal colon of the rabbit

Four types of contractile activity were identified and characterised in the isolated triple haustrated proximal colon of the rabbit using high-definition spatiotemporal mapping techniques. Mass peristalses were hexamethonium-sensitive deep circular contractions with associated taenial longitudinal contractile activity that occurred irregularly and propagated rapidly aborad, preceded by a zone of local lumen distension. They were sufficiently sustained for each event to occupy the length of the isolated colonic segment and the contraction persisted longer orally than aborally, the difference being more pronounced when lumen contents were viscous. Haustra were bounded by deep even-spaced ring contractions that progressed slowly aborad (haustral progression). Haustral formation and progression were hexamethonium-sensitive and coordinated across intertaenial domains. Ripples were hexamethonium-resistant phasic circular contractions that propagated predominantly orad at varying rates. In the presence of haustra, they were uncoordinated across intertaenial domains but were more coordinated when haustra were absent. Fast phasic contractions were relatively shallow hexamethonium-resistant contractions that propagated rapidly in a predominantly aborad direction. Fast phasic circular contractions were accompanied by taenial longitudinal muscle contractions which increased in amplitude prior to a mass peristaltic event and following the administration of hexamethonium. On the basis of the concurrence and interaction of these contractile activities, we hypothesise that dual pacemakers are present with fast phasic contractions being modulated by the interstitial cells of Cajal in the Auerbach's plexus (ICC-MY) while ripples are due to the submucosal ICC (ICC-SM). Further, that ICC-SM mediate the enteric motor neurons that generate haustral progression, while the intramuscular ICC (ICC-IM) mediate mass peristalsis. The orad movement of watery fluid was possibly due to ripples in the absence of haustra.

Internal free magnesium modulates the voltage dependence of contraction and Ca transient in rabbit ventricular myocytes.

We investigated the effect of altering internal free magnesium concentration (Mgi) on the contraction and Cai transient of patch-clamped rabbit ventricular myocytes. Experiments were performed at 35 degrees C; cells were held at -40 mV to inactivate Na channels and T-type Ca channels, and at this potential (and in the absence of cyclic AMP) "Ca-induced Ca release" is the primary trigger mechanism. Cells dialysed with a low Mgi (2.9 muM) had a large and fast phasic contraction and Cai transient at positive potentials (+60, +80 mV). Cells dialysed with a high Mgi (7.1 mM) had a small or absent phasic contraction and Cai transient at positive potentials. These effects were due to a change in free Mgi, and not due to a change in [Mg.ATP]. In cells dialysed with a low Mgi, application of Ca channel blockers (32 muM nifedipine with 10 muM D600) for a single beat abolished current through L-type Ca channels (ICa,L); however, 53% of the Cai transient was still elicited. Adding 5 mM Ni to Ca channel blockers abolished the remaining Cai transient, indicating that (in the absence of ICa,L) the transient might be triggered by reverse Na/Ca exchange. In cells dialysed with a high Mgi, a single-beat switch to Ca channel blockers was sufficient alone to abolish the Cai transient, indicating that under these conditions Ca entry via ICa,L is the primary sarcoplasmic reticulum trigger mechanism. These results suggest that raised free Mgi might partially inhibit the activity of the Na/Ca exchange, or might limit its ability to trigger Ca release.

The effect of Mn 2+, Zn 2+, Ba 2+ and Ca 2+ on spontaneous motility in sheep duodenum in vitro

1. 1. The effects of several ions, Mn 2+, Zn 2+, Ba 2+ and Ca 2+, on spontaneous motility were investigated in longitudinal smooth muscle strips from sheep duodenum, in vitro 2. 2. Mn 2+ (0.5–1.5 mM) and Zn 2+ (0.5–5 mM) inhibited both the amplitude and frequency of motility in Krebs solution and in Ca 2+-free medium. 3. 3. Ba 2+(0.5–5 mM) evoked three types of contractile responses: (i) an increase in the frequency and a reduction of the amplitude of spontaneous contractions; (ii) a slight increase in muscle tone of the phasic contractions; and (iii) a rapid initial phasic contraction followed by slowly fading contraction. Ca 2+ induced two kinds of responses in spontaneous motility: (i) a fast phasic contraction, followed by an increase in the amplitude and frequency of phasic contractions with no changes in its tone; and (ii) an increase in the amplitude of contractions. 4. 4. The Ba 2+-induced contractions were inhibited by EDTA, verapamil and diltiazem, but were not modified by sodium nitroprusside. The Ca 2+-induced contractions were reduced by verapamil and diltiazem. 5. 5. Our results show that Mn 2+ and Zn 2+ behave as inhibitors of sheep duodenum motility. In contrast, Ba 2+ and Ca 2+ stimulate motility. It is suggested that Ba 2+ can penetrate the cells through voltage-dependent Ca 2+ channels and behave as a partial substitute for Ca 2+.

Electrical and chemical control of smooth muscle activity of rabbit corpus cavernosum in vitro

This study sought to further elucidate the regulation of cavernous smooth muscle tone and to characterize mechanisms of cavernous activation and relaxation. In isolated strips of rabbit corpus cavernosum, extracellular electrical and mechanical activity were recorded simultaneously before and after pharmacologic stimulation. Spontaneous mechanical activity was characterized by fast phasic contractions (frequency 6 to 30 min-1) associated with fluctuations of the extracellular electrical signals. Phasic activity was increased by blockade of potassium channels or by moderate activation of L-type calcium channels. Faster spikelike fluctuations occurred in the electrical activity, indicating the existence of spike discharges. All mechanical and electrical fluctuations were completely abolished by blockade of L-type calcium channels with nifedipine. Our results indicate that cavernous smooth muscle tone is regulated by both phasic and tonic activation mechanisms caused by the opening of L-type calcium channels and calcium influx through chemically controlled calcium influx/release.

Effects of cholecystokinin on chicken cecal motility: mechanisms involved.

The aims of this work are to characterize the effects of cholecystokinin (CCK) on chicken ceca and to study in vitro the mechanisms through which such actions are mediated. Longitudinal and circular cecal strips kept in vitro in organ baths were responsive to CCK sulphated octapeptide (CCK-8s). On longitudinal strips the response consisted of a fast phasic contraction followed by a sustained increase in tone which was dose dependent and decreased markedly in the presence of tetrodotoxin (TTX). Ketanserin (10(-5) M) also caused a decrease in the CCK-8s response. CCK tetrapeptide (CCK-4) and CCK unsulphated octapeptide (CCK-8ns) induced slightly less contractile effects at concentrations of 2 x 10(-6) M only. L365,260 and L364,718 decreased the response of longitudinal strips to CCK-8s with similar efficacy. On circular strips CCK-8s caused rhythmic phasic contractions of dose dependent amplitude and frequency, and both effects were resistant to TTX. The EC50 for the amplitude was about 4 times higher than that for the frequency. CCK-8ns (2x 10(-6) M) also caused phasic contractions, whereas the same concentrations of CCK-4 did not elicit any motor effects. L365,260 and L364,718 showed different efficacy in decreasing amplitude or frequency of contraction. These results suggest that 1) Both muscularly and neurally located CCK receptors are present on the longitudinal layer of chicken ceca whereas only muscular receptors are present on the circular muscle. 2) 5HT2 receptors seem to be involved in the neurally mediated CCK-8s response observed in the longitudinal layer. 3) The different potency of CCK-8s, CCK-8ns and CCK4 to induce contractile effects and of the CCK-A and CCK-B antagonists to block such effects suggests the existence of two different CCK receptors on the circular layer.

Amplifying effect of serotonin on contractile responses in rat aorta and depletion of intracellular Ca-stores

1. 1. Serotonin, 1 μM, induces a contractile response in isolated rat aorta in the presence or absence of extracellular Ca. 2. 2. In Ca-free media, the fast phasic contraction is lower in magnitude and further addition of serotonin evokes no response. 3. 3. Recovery of the contractile response in Ca-free medium is obtained by a 40 min incubation in Ca-containing solution. 4. 4. In Ca, Mg-free medium, the response to serotonin is significantly higher than that obtained in the presence of Mg. 5. 5. An amplifying effect of serotonin on the contractile responses induced by serotonin itself or by noradrenaline was observed in Ca-containing but not in Ca-free solution.

Different and common intracellular calcium-stores mobilized by noradrenaline and caffeine in vascular smooth muscle.

Noradrenaline (NA) 1 microM and caffeine (CAF) 10 mM induce a contractile response in isolated rat aorta maintained at 37 degrees C either in the presence or absence of extracellular calcium. In Ca-free media the contractile response was reduced and contractile activity of CAF only occurred at 25 degrees C. NA induced a biphasic response in Ca-free medium, with a fast phasic contraction followed by a smaller more sustained contraction. The response induced by CAF consisted of a fast transient contraction which returned to a level below the resting tone. After washing, further addition of NA or CAF evoked no increase in smooth muscle tension. The influence of Mg-depletion in the extracellular medium on the contractile responses induced by NA and CAF in Ca-free medium was determined: similar response to NA or CAF in media with or without Mg after 2 min loading were observed, but after 15 min loading in Mg, Ca-free solution, the responses to NA or CAF were significantly higher than after incubation in Ca-free medium containing Mg. Differences were observed when contractile response to NA was elicited after the refilling process by loading the aortic strip in Ca-containing Mg-free solution. In this case, instead of recovering the magnitude of contraction there was a significant decrease. The existence of two independent intracellular Ca-pools releasable by NA, one of them also sensitive to CAF, is postulated. The refill of the Ca-store specific to NA is dependent on the presence of Mg in the extracellular medium. In contrast, the refill of the common Ca-pool releasable by NA and CAF is independent of the extracellular Mg, but its spontaneous emptying is temperature-, and Mg-dependent. This suggests the intermediacy of an enzymatic process to extrude the Ca-content of this store to the extracellular space.

Prostaglandin E 2 selectively affects purinergic transmission in guinea pig vas deferens

The effect of prostaglandin E 2 (PGE 2) on the contractile response of the guinea pig vas deferens was examined. Postganglionic hypogastric nerve stimulation for 7 sec at 20 Hz induced a biphasic contractile response, consisting of fast phasic and delayed tonic components. Prostaglandin E 2 delayed the onset and increased the maximum contractile responses. Stimulation in the presence of prazosin induced only a fast phasic contraction. Treatment with PGE 2, in the presence of prazosin, delayed the onset of this response and increased its maximum. The delayed contraction, observed on stimulation in the presence of α,β-methylene adenosine triphosphate (ATP), was enhanced moderately and concentration-dependently by PGE 2. Short-term stimulation with 5 pulses induced a small fast phasic contraction. This contraction, which could be desensitized by α,β-methylene ATP, was inhibited by PGE 2 but not by prazosin. Prostaglandin E 2 significantly enhanced the transient phasic contraction, induced by addition of exogenous ATP to the organ bath and had a similar but somewhat smaller effect on the tonic contraction induced by the addition of exogenous norepinephrine (NE). These findings suggest that PGE 2 selectively delayed neurotransmission, mediated by ATP and enhanced contractions of the smooth muscle of guinea pig vas deferens, elicited by ATP or NE.

Simulation of K-contracture curve in smooth muscle (author's transl)

40 mM K-induced isometric contracture of guinea pig taenia coli in the presence of 30 microM dantrolene showed a rapidly rising peak followed by a plateau and then a low sustained tonic contraction. The plateau was delayed by low Ca so that two phasic contractions, fast and slow, were separated from each other. In K-contracture after 15 sec contact with normal Ca following low Ca condition, the fast phasic contraction but not the slow one regained its tension depressed by preceding low Ca, while the slow phasic contraction recovered from its delay. In the presence of 0.2 microM verapamil, K-contracture consisted of the fast phasic contraction without plateau and of the low tonic contraction. The results suggest that K-contracture in the normal state consists of three components, the fast and slow phasic contractions and the tonic contraction, and that dantrolene inhibits a tonic contraction, whereas verapamil inhibits the slow phasic and the tonic contraction. Thus, the contraction curve of each component was tentatively expressed by exponential function, and for simulation of the observed curves a computer was utilized to synthesize the curves using the components of varying parameters. The simulation was successful when based on the above suggestion.