Guinea-pig Ileal Longitudinal Smooth Muscle Research Articles

Palytoxin-induced K+ efflux from ileal longitudinal smooth muscle of the guinea-pig.

In guinea-pig ileal longitudinal smooth muscle, both palytoxin (PTX) and carbachol (CCh) increased K+ efflux with an EC50 of 1.8 X 10(-10) M and 4.1 X 10(-7) M, respectively. Atropine (10(-6) M) did not inhibit the K+ efflux due to PTX (3 X 10(-9) M), but completely inhibited the efflux due to CCh (10(-5) M). External Ca2+ removal and verapamil (10(-5) M) did not change the PTX-induced K+ efflux, although the CCh-induced K+ efflux was inhibited about 77% and 71%, respectively. PTX-induced K+ efflux was reduced to 31% by a depletion of intracellular Ca2+. Tetraethylammonium (15 mM) inhibited the K+ efflux due to PTX or CCh to 61% or 75%, respectively. The PTX-induced K+ efflux was also inhibited by cymarin (3 X 10(-8) M), ouabain (10(-5) M) and digitoxin (10(-5) M). These results suggest that the PTX-induced K+ efflux is less dependent on Ca2+ influx than that due to CCh. Furthermore, the binding sites for PTX in the ileal muscle of guinea-pig may be Na+, K+-ATPase, as has been suggested in other types of cells.

Palytoxin-lnduced K+ Efflux from Ileal Longitudinal Smooth Muscle of the Guinea-Pig

In guinea-pig ileal longitudinal smooth muscle, both palytoxin (PTX) and carbachol (CCh) increased K+ efflux with an EC50 of 1.8×10−10 M and 4.1 ×10−7 M, respectively. Atropine (10−6 M) did not inhibit the K+ efflux due to PTX (3×10−9 M), but completely inhibited the efflux due to CCh (1Cr5 M). External Ca2+ removal and verapamil (10−5 M) did not change the PTX-induced K+ efflux, although the CCh-induced K+ efflux was inhibited about 77% and 71%, respectively. PTX-induced K+ efflux was reduced to 31% by a depletion of intracellular Ca2+. Tetraethylammonium (15 mM) inhibited the K+ efflux due to PTX or CCh to 61% or 75%, respectively. The PTX-induced K+ efflux was also inhibited by cymarin (3×10−8 M), ouabain (10−5 M) and digitoxin (10−5 M). These results suggest that the PTX-induced K+ efflux is less dependent on Ca2+ influx than that due to CCh. Furthermore, the binding sites for PTX in the ileal muscle of guinea-pig may be Na+,K+-ATPase, as has been suggested in other types of cells.

Benzodiazepine interactions at neuronal and smooth muscle Ca 2+ channels

Ro 5-4864 and PK 11195, ligands for the peripheral benzodiazepine binding site, blocked 45Ca 2+ uptake into depolarized guinea-pig cortical synaptosomes. The fast phase of uptake was significantly more sensitive than the slow phase. The central benzodiazepine ligands, Ro 15-1788 and β-CCE at 10 −4 and 5 × 10 −5 M respectively, were less effective and did not discriminate between fast and slow uptake phases. Ro 5-4864 and PK 11195 inhibited and potentiated respectively the binding of [ 3H]nitrendipine to synaptosomes by effects on the K D value. PK 11195 (10 −5 M), blocked K + depolarization and Bay K 8644-induced tension responses in guinea-pig ileal longitudinal smooth muscle. Benzodiazepines do interact with voltage-dependent Ca 2+ channels but the present data are insufficient to prove direct coupling of peripheral benzodiazepine sites with Ca 2+ channels.

Specific and non-specific desensitization of guinea-pig ileal smooth muscle.

The effects of cis-2-methyl-4-dimethylaminomethyl-1-3-dioxolane methiodide (CD), a muscarinic agonist, histamine, substance P and K+-stimulation on the mechanical responses, Ca2+-dependence and desensitization in guinea-pig ileal longitudinal smooth muscle have been studied. The mechanical responses to all four stimulants are highly dependent upon extracellular Ca2+(Ca2+EXT) and are blocked by the Ca2+ channel antagonist nicardipine. The tonic (slow) components of response are more dependent on Ca2+EXT and are more sensitive to nicardipine (IC50 values 5.0 X 10(-8) - 2.5 X 10(-9)M) than are the phasic (fast) components of response. Tissue exposure to CD (5 X 10(-7)M, 10 min) or histamine (3 X 10(-6)M and 3 X 10(-4)M, 10 min) produces short term nonspecific desensitization but substance P (5 X 10(-8)M, 10 min) produces only specific desensitization. K+-induced responses neither desensitize nor are desensitized. Desensitization is concentration- and time-dependent for both specific and nonspecific processes. Nonspecific desensitization is protected by elevation of K+ concentration (5.36mM) in the incubating medium, by dithiothreitol and by inhibitors (mepacrine,p-bromophenacyl bromide and phenylgloxal) of phospholipase A2 and is potentiated by mellitin, an activator of phospholipase A2. Desensitization produced by the muscarinic agonist CS is protected by Gpp(NH)p (10(-4)M), but histamine-induced desensitization is unaffected. There is no loss of muscarinic receptors, measured by [3H]QNB binding following tissue exposure to low concentrations of CD (5.0 X 10(-7)M) for up to 72 h. However, an apparent loss of receptors (20-30%) is measured following 10-90 min exposure of tissue to 10(-3)M CD. It is suggested that contractions of guinea-pig ileal longitudinal smooth muscle elicited by CD, histamine, substance P or K+ mobilize a common pool of Ca2+ through a Ca2+ channel antagonist (nicardipine) sensitive pathway. However, the existence of short term nonspecific desensitization (CD and histamine), specific desensitization (substance P) or no desensitization (K+ stimulation) indicates that significant differences exist in the pathways linking initial stimulus to mechanical response. The ability of elevated K+ to protect against nonspecific desensitization suggest that post stimulus membrane hyperpolarization may represent one contributing component to nonspecific desensitization. Products of phospholipid degradation may also contribute to desensitization since inhibitors or activators of phospholipase A2 prevented or potentiated respectively, nonspecific desensitization.

The action of the ionophore ionomycin in guinea-pig intestinal smooth muscle.

The ionophore ionomycin produced concentration-dependent (5 X 10(-9) to 5 X 10(-6) M) contractions in guinea-pig ileal longitudinal smooth muscle. Responses were dependent on extracellular Ca2+, consistent with the known role of this Ca2+ source in supporting excitation-contraction coupling to a variety of stimulants in this tissue. Responses were insensitive to atropine (10(-6) M) but were dependent upon extracellular Na+ and were completely blocked by low concentrations of the Ca2+-channel antagonists nicardipine, YC-93 (5 X 10(-7) M), and D-600 (5 X 10(-6) M). The behaviour of ionomycin is very similar to that shown by A 23187 in this tissue. Ionomycin, like A 23187, can apparently activate D-600 sensitive Ca2+ channels in the guinea-pig intestinal smooth muscle rather than simply translocating Ca2+EXT.

BARIUM-INDUCED CONTRACTIONS AND INHIBITORY ACTIONS OF D-600, BENACTYZINE AND PAPAVERINE ON THEIR CONTRACTIONS IN THE GUINEA-PIG ILEAL LONGITUDINAL SMOOTH MUSCLE

In the guinea-pig ileal Longitudinal smooth muscle, Ba caused an initial phasic contraction, followed by a gradual decrease of the contraction (tonic contraction). In the Ca-free and the Ca-free, K-rich physiological solution, Ba could also contract the longitudinal muscle, but 30min- and 60min-immersion of the tissue in the Ca-free solution decreased lower than 20% of control of the Ba contraction. Although the Kinduced depolarization intensified the Ba contraction after the 30min-immersion, within 10min after the immersion in the two solutions there is no difference in the height of Ba contractions. In the normal physiological solution, Ba seems to serve the contractile protein with free Ca ions rather than itself for the Ba contraction. D-600 induced an equal inhibition on the Ba contractions in the normal physiological solution and in the Ca-free solution, and it inhibited the tonic contraction more than the phasic one. The phasic component may be due to the cellular Ca mobilized by Ba which passes through D-600-sensitive pathway, and at least part of the tonic component may utilize the extracellular Ca. Bencetyzine perferentially inhibited the Ba contraction in the Ca-free solution, and it depressed the phasic component more than the tonic one. Treatment with papaverine induced an equal inhibition on the phasic and tonic components of Ba contractions and the Ba contraction in the Ca-free solution .Benactyzine may exert the main influence on the mobilization of the cellular Ca through the competition with Ca. Papaverine seems to inhibit the Ba contractions by diverse actions.

Calcium and the muscarinic receptor

Pharmacological receptors may be viewed as composed of two linked functions, a recognition site through which the specificity and selectivity of ligand action is expressed and an amplification or catalytic site which translates the ligand-recognition site interaction into response. The successful elucidation of receptor mechanisms requires analysis of both sites. The muscarinic cholinergic receptors of guinea-pig ileal longitudinal smooth muscle will be discussed from this two function stand point and the source and utilization of Ca 2+ in excitation-contraction coupling discussed.

An analysis of the action of cations of the lanthanide series on the mechanical responses of guinea-pig ileal longitudinal muscle.

1. The inhibitory effects of lanthanide cations (Ln3+) on mechanical responses and 45Ca uptake in guinea-pig ileal longitudinal smooth muscle were studied. 2. Ln3+ strongly inhibited the phasic and tonic component of the response to the muscarinic agonist cis-2-methyl-4-dimethylaminomethyl-1,3-dioxolane methiodide (CD) the two components being affected to the same extent. Inhibition was also obtained for the responses evoked by high K+ but here the effect was mainly on the phasic response, the tonic component was merely delayed. 3. Other members of the Ln3+ series, with the exception of cerium, were found to be more effective than lanthanum in their ability to inhibit the CD response. Thulium, Tm3+, the thirteenth member of the series was the most effective cation. 4. Analysis of 170Tm uptake revealed at least two components. The concentration-dependence of one component, saturating at 2-5 x 10(-6) Tm, corresponded closely to that of the inhibitory effect of Tm3+ on contraction. 5. 170Tm uptake as a function of time showed a secondary rise after 30 min of exposure to the lanthanide. 6. Although 2-5 x 10(-6) M-Tm3+ produced 90% inhibition of the CD and the high K+ induced responses significant reduction of 45Ca uptake by the muscle was only detected when much higher Tm3+ concentrations (greater than or equal 10(-3) M-Tm3+) were used. 7. It is concluded that Ln3+ combine with membrane sites specifically involved in Ca2+ translocation during excitation-contraction coupling.

The action of the ionophores, X-537A and A-23187, on smooth muscle.

The cation ionophore X-537A in the concentration range of 10(-6) to 3 X 10(-5) M produced contractions in the rat and guinea-pig vas deferens. No contractile effect was produced in either of the vasa deferentia preparations by the ionophore A-23187 in the concentration range of 10(-7) to 5 X 10(-5) M. In contrast, X-537A had no contractile effect on the guinea-pig ileal longitudinal smooth muscle while A-23187 produced a dose and [Ca2+] dependent contraction. The contractile effect of X-537A in the vasa deferentia preparations is abolished by phenoxybenzamine or prior reserpine treatment and is therefore attributed to the release of norepinephrine. The effect of A-23187 in the intestinal smooth muslce is attributed to a direct Ca2+ transporting action since its contractile effect is unaffected by histamine, acetylcholine, or 5-hydroxytryptamine antagonists.