Membrane potential changes associated with tachyphylaxis and potentiation of the response to stimulating drugs in smooth muscle

Differentiation of the action potential in the smooth muscle of canine gastric antrum using calcium-inhibitory drugs

Electrical and mechanical activity were recorded simultaneously in smooth muscle preparations from the antrum region of canine stomach by means of a single sucrose gap technique (SGT). The SGT was optimized to permit stable recording from multicellular smooth muscle preparations over several hours of electrical and mechanical activity with little disturbance of their normal properties. Acetylcholine (ACh, 10(-8) to 10(-6) M) induced or augmented dose-dependently the electrical and mechanical activity. The plateau of the action potential complex was elevated by ACh, while the contraction was increased in linear correlation to the magnitude of the plateau component. In spontaneously active (or in ACh-stimulated) preparations TEA (5 to 20 mM) magnified the plateau component, induced or strengthened spikes on the plateau ('secondary spikes'), and induced or strengthened phasic contractions. Nifedipine (10(-6) M) abolished secondary spikes, part of the plateau component of the action potential, and suppressed mechanical activity. The complex action potential of canine gastric antrum can be differentiated into (a) a basic action potential, consisting of an initial, primary spike and a plateau depolarization; this basic action potential is resistant to nifedipine and does not trigger any mechanical activity; and (b) a nifedipine-sensitive component (calcium component), which consists of an augmentation of the plateau depolarization and of secondary spikes, and which is responsible for the initiation of mechanical activity.

Comparison of the excitatory actions of substance P, carbachol, histamine and prostaglandin F2 alpha on the smooth muscle of the taenia of the guinea-pig caecum

A comparison was made of the actions of substance P, prostaglandin F2 alpha (PGF2 alpha), histamine and carbachol on the membrane potential and conductance of the longitudinal smooth muscle of the taenia of the guinea-pig caecum using the double sucrose gap apparatus. The increases in conductance produced by the four drugs during matched depolarizations in the sucrose gap were not significantly different and they were substantially larger than the increase in conductance brought about during the same depolarization produced by passing outward current. In sucrose-substituted, Na- and Cl-deficient solution the increases in conductance and depolarization due to carbachol, substance P, and PGF2 alpha were attenuated to a similar extent. The depolarization due to histamine under these conditions was reduced to a significantly greater extent than that due to carbachol. In Tris-benzene-sulphonate substituted Na- and Cl-deficient solution the responses due to carbachol and histamine were attenuated to a similar extent. This suggests that sucrose addition may have a specific effect on the histamine response. In Tris-substituted Na-deficient solution the increases in conductance and depolarization produced by substance P, histamine and carbachol were attenuated to a similar extent. The depolarization due to PGF2 alpha was reduced by a significantly greater amount which may be due to unmasking an inhibitory effect that was sometimes apparent in normal solution. In benzenesulphonate-substituted, Cl-deficient solution the increases in conductance and depolarizations produced by moderate concentrations of PGF2 alpha, histamine and carbachol were attenuated to a similar extent. The response to substance P was little affected. In glucuronate-substituted, Cl-deficient solution the increases in conductance and depolarizations due to substance P and carbachol were attenuated to a similar extent. This result, and the observation that the depolarization to large concentrations of carbachol was not reduced in benzenesulphonate-substituted, Cl-deficient solution, suggest that benzenesulphonate interferes with the reactions of the non-peptide stimulants with their respective receptors. The similarities in the effects of activating the four types of receptor under some conditions could be explained if they all acted on the same population of receptor-operated channels. In addition it seems that PGF2 alpha acts also on a population of inhibitory receptors, sucrose interferes with histamine's action, and benzenesulphonate interferes with the reactions of non-peptide stimulants with their respective receptors.

Alteration of smooth muscle contractility after muscarinic agonist-induced K+ loss

1. After stimulation of the longitudinal smooth muscle of the guinea-pig ileum by an optimal dose (2 x 10(-7) M) of a muscarinic agent, cis-2-methyl-4-dimethylaminomethyl-1,3-dioxolane methiodide (CD), the muscles failed to regain their normal spontaneous activity for 20 to 30 min. During the recovery period, subsequent contractions induced by either CD or 60 mM KCl were altered, particularly when only short times (15 min or less) were allowed between exposures. 2. Altered responses to CD had depressed phasic but increased tonic tensions and were characteristic of responses induced by lower doses of CD. The altered responsiveness probably represented an early phase of muscle 'densensitization'. 3. In contrast to muscarinic stimulation, the smooth muscles gave identical responses after repeated stimulation by 60 mM KCl, even when only 2 min were allowed between exposures. 4. Whereas K+ levels increased in muscles exposed to 60 mM KCl, they decreased during contractions to CD. The K+ levels remained low until the muscles recovered their normal responsiveness. 5. Increasing the extracellular K+ concentration (5 to 13 mM) hastened the recovery of the muscle responsiveness after CD, whereas lowering external K+ concentration to 1.35 mM or the addition of ouabain (5 x 10(-7) M) delayed the recovery. The results suggested that the Na+, K+-pump is rate-limiting in the recovery of the normal ionic balance of the muscles after stimulation by muscarinic agonists.

Characteristics of histamine tachyphylaxis in canine tracheal smooth muscle

In isolated canine tracheal smooth muscle, repeated administrations of histamine result in a rapid reduction in contractile response to about 15% of the initial contraction (tachyphylaxis). Development of this tachyphylaxis is specific inasmuch as: 1) it does not develop to acetylcholine (10(-6) M or 10(-4) M), or serotonin (10(-5) M; and 2) maximally developed histamine tachyphylaxis is not associated with a parallel reduction in response to acetylcholine. Pretreatment with propranolol (10(-5) M) or phentolamine (10(-4) M) does not prevent tachyphylaxis: however, pretreatment with atropine (10(-4) M) does prevent tachyphylaxis in about 50% of the animals tested. Tachyphylaxis to histamine can be reversed in a dose- and time-dependent fashion with prostaglandin synthesis inhibiting agents. The order of potency obtained with such compounds (indomethacin greater than mefenamic acid greater than oxyphenbutazone greater than acetylsalicylic acid) is consistent with potencies for inhibition of prostaglandin synthesis found in the literature. Also, in indomethacin pretreated strips in which tachyphylaxis to histamine was prevented, exogenous addition of PGE2 (1.42 x 10(1-) M to 2.84 x 10(-9) M) and PGA2 in a high concentration (2.9 x 10(-9) M) are capable of selectively reducing the response to histamine without an effect on acetylcholine-induced contractions. These data suggest that the mechanism of histamine tachyphylaxis in the canine tracheal smooth muscle preparation involves prostaglandin synthesis.

The action of 5-hydroxytryptamine on chemoreceptor discharges of the cat's carotid body

1 Chemoreceptor discharges were recorded in vivo from fine filaments of the carotid sinus nerve containing a single or several active units; their frequency was used as an index of receptor activity. The effects of 5-hydroxytryptamine (5-HT) on chemoreceptors were studied in 26 adult cats. At times, sinus baroreceptor discharges were recorded from the carotid nerve and the effect of 5-HT on the discharges was examined. 2 Intra-carotid injections of 5-HT (2-20 mug) induced a sharp and brief increase in chemoreceptor discharges, followed by depression or block which lasted for several seconds. Repeated injections at short intervals, and a small dose after a large dose of 5-HT resulted in depressed or blocked response to 5-HT. 3 5-HT in high doses (10-20 mug, i.a.) slightly depressed the chemoreceptor discharges induced by either acetylcholine (ACh) or NaCN, when these substances were applied within 20 s after 5-HT. 5-HT (5-20 mug, i.a.) applied during asphyxia induced a further increase in chemoreceptor discharges, soon followed by block of the discharges lasting for several seconds. 4 Atropine or hexamethonium in high doses did not change the chemoreceptor response to 5-HT, while that to ACh was markedly depressed. 5 (+)-Lysergic diethylamide (LSD), methysergide or gramine did not alter the response to 5-HT, while LSD in low doses produced a marked increase in chemoreceptor discharges. 6 Acute and chronic treatment with reserpine (5-10 mg/kg, i.v.) of the animals did not change the sensitivity and the reactivity of the chemoreceptor to ACh and NaCN, while the chemoreceptor response to 5-HT was augmented, indicating an increase in the sensitivity of chemoreceptors to 5-HT. 7 5-HT in small doses (2-10 mug, i.a.) induced a marked increase in sinus baroreceptor discharges; subsequently discharges were depressed or blocked for several seconds. 8 The results are discussed in relation to possible mechanism of action of 5-HT on the chemoreceptors. It is concluded that the exogenous 5-HT probably acts directly on the chemosensory nerve endings and depolarizes them, but 5-HT contained in the carotid body does not play a significant role in the generation of chemoreceptor discharges.

The effect of histamine on the electrical activity of rat uterus

Histamine produces a reduction in the spike frequency and the degree of depolarization which accompany spontaneous and induced contractions of rat uterus. Concentrations of 0.5 mug/ml can cause complete inhibition of both mechanical and electrical activity without producing any change in the resting membrane potential. Concentrations as high as 100 mug/ml cause no appreciable change in membrane potential.

The spontaneous electrical and mechanical activity of the longitudinal smooth muscle of the rabbit duodenum and its modification by drugs and temperature changes

The electrical and mechanical activity of strips of longitudinal smooth muscle from the rabbit duodenum was recorded using the sucrose gap method. The muscle exhibited rhythmic tension changes each of which was associated with a slow potential wave of electric depolarization surmounted by a burst of spike activity (a multispike complex). Cooling the tissue reduced the frequency of tension waves and multispike complexes. Catecholamines produced a reduction in the amplitude of tension waves. This was associated with hyperpolarization and decreased spike activity. Slow wave frequency and amplitude were unaffected. Acetylcholine, methacholine and histamine produced tension wave fusion associated with depolarization and increased spike activity.

Osmolarity of urine from renal pelvis and bladder of foetal and post-natal pigs

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Tetrodotoxin and manganese ions: effects on electrical activity and tension in taenia coli of guinea pig

Tetrodotoxin, at concentrations up to 5 x 10(-6) gram per milliliter, has no effect on the spontaneous discharge in the smooth muscle of taenia coli. However, the spontaneous discharge is abolished by Mn(++) at a concentration of 0.5 millimole per liter. The contraction induced by immersing the muscle in isotonic KCl solution is also suppressed in the presence of Mn(++). Because Mn(++) is a specific suppressor of the spike induced by Ca(++) and tetrodotoxin is an inhibitor of the spike induced by Na(+), we suggest that Ca(++) is a charge carrier in the production of spike potential in the smooth muscle and that the entry of intervening Ca(++) through the membrane acts as a trigger for the contraction of smooth muscle.