Excitation-contraction coupling in multiunit tracheal smooth muscle during metabolic depletion: induction of rhythmicity

Limb immobilization alters muscle activation patterns during a fatiguing isometric contraction

The purpose of the study was to determine the role of excitation-contraction coupling in the increased endurance time for low-force contractions after 4 weeks of elbow joint immobilization. Twelve subjects participated in a protocol that required immobilization of the elbow joint in a fiberglass cast for 4 weeks, and 4 subjects acted as controls. Measurements of muscle strength, contractile properties, and fatigability were performed before and after 4 weeks of limb immobilization, and after 4 weeks of recovery. The immobilization intervention produced significant reductions in the daily activity of the elbow flexor muscles, a 21% decline in the maximum voluntary contraction (MVC) force, and a 31% decrease in the maximum load that could be lifted once. Seven of the immobilized subjects exhibited an unusual pattern of muscle activity during the fatiguing contraction after immobilization, which was associated with an increase in the endurance time of the elbow flexor muscles (mean = 220%) in these subjects. The unusual pattern of muscle activity involved lower relative activity of the brachialis muscle, no increase in the amplitude of the electromyogram (EMG) for the elbow flexor muscles, and intermittent rather than continuous EMG. In contrast, the force-frequency relationship of biceps brachii was not altered by immobilization in these subjects, suggesting that adaptations in excitation-contraction coupling were not the primary cause of the prolonged endurance time after immobilization. Rather, the results suggest that the prolonged endurance time exhibited by some subjects after immobilization was largely due to adaptations within the nervous system.

Na+/K+ ATPase mediates rhythmic spontaneous relaxations in canine airway smooth muscle

In canine airway smooth muscle, cyclopiazonic acid (CPA; selective blocker of the sarcoplasmic reticulum Ca(2+)-pump) evokes a contractile response which is initially mediated via release of internally sequestered Ca2+, but is later supported almost exclusively by electromechanical coupling. As such, this second component is highly sensitive to inhibition of voltage-dependent Ca(2+)-influx (e.g. dihydropyridines, removal of external Ca2+) or to membrane hyperpolarization. In the present study, we describe relaxations which occur spontaneously during this second component of the CPA-evoked contraction. These relaxations are also electromechanically mediated, since they are abolished by depolarization of the membrane by high-K+ media. TEA has relatively little effect on the phasic activity, thus ruling out an involvement of Ca(2+)-dependent K+ channels. On the other hand, the phasic activity is abolished by ouabain, by removal of external K+, or by cooling and is markedly slowed by removal of external Na+. These observations indicate that the phasic activity is mediated by the Na+/K(+)-ATPase.

Identification of aldosterone secretion inhibitory factor in bovine adrenal medulla

We report the first demonstration of an Aldosterone Secretion Inhibitory Factor (ASIF) in acid extracts of bovine adrenal medulla. Following separation from catecholamines and enkephalins, this factor leads to an 80% inhibition of PGE1-stimulated secretion of aldosterone from bovine adrenal zona glomerulosa. ASIF is retained on cation exchange gels and behaves as a small 5K-dalton peptide on Sephadex G-50. This factor cross-reacts in a radio-receptor assay for [125I] atrial natriuretic factor (ANF). ASIF is distinct from all neuropeptides formerly detected in the adrenal medulla, e.g. somatostatin, enkephalin, neuropeptide Y, dynorphin, neurotensin. In the adrenal gland, this ANF-like factor is predominantly found in the medulla (4 pmol/mg protein), with only trace amounts in the cortex (0.1 pmol/mg protein). ASIF might perhaps correspond to the endogenous ligand for the receptor sites that we have previously identified with [125I]ANF in bovine adrenal cortex and could contribute to the formerly reported attenuating influence of the adrenal medulla on mineralocorticoid production.

Histamine-induced rhythmic contraction of hog carotid artery smooth muscle

Smooth muscle strips isolated from the hog common carotid artery can contract rhythmically, exhibiting low frequency, large amplitude oscillations in tension when stimulated with 10 microM histamine. Strips required at least 1.45 mM calcium and 2.5 mM potassium to exhibit this rhythmic activity. Rhythmic contractions could be converted to tonic contractions by removal of potassium or ouabain treatment. Relaxation by 2 mM lanthanum, 1 mM manganese, or 1 microM verapamil implies that the external medium is the source of calcium mediating the contractions. The involvement of adrenergic nerve terminals in this response was ruled out, since propranolol, phentolamine, tetrodotoxin, bretylium, or 6-hydroxydopamine treatment did not alter the oscillations. Blockade of H1 receptors with 0.1 microM diphenhydramine relaxed the muscle strips. The H2 receptor antagonist cimetidine (5 microM) had no effects. Attempts to obtain rhythmic contractions by stimulating with other vasoactive agents (norepinephrine, acetylcholine, 5-hydroxytryptamine, angiotensin II, and elevated potassium concentrations) were unsuccessful, suggesting that this is a specific histamine response mediated solely by H1 receptors. These results show that this large artery, commonly considered a multi-unit smooth muscle, can sometimes exhibit single-unit behavior.

Studies on an isolated innervated preparation of guinea-pig trachea

An isolated preparation of the guinea-pig trachea with intact parasympathetic and sympathetic innervation has been devised. Responses to nerve stimulation were recorded as increases or decreases in intraluminal pressure from the fluid-filled trachea. The preparation maintained a positive resting intraluminal pressure of 3-4 cmH2O. This was unaffected by atropine, hexamethonium or propranolol. Brief pressor responses, which could be completely blocked by atropine or hexamethonium, were obtained by applying short trains of stimuli to the cervical segment of the right vagus. The amplitude of responses was frequency-dependent up to a maximum at 40 Hz. Depressor responses, more delayed and prolonged than the pressor responses and blocked by propranolol but not by hexamethonium, were obtained by stimulation of the right cervical sympathetic trunk or stellate ganglion in 70% of preparations. Dual pressor-depressor responses were observed in the remaining 30% of preparations. The pressor component was blocked by atropine, the depressor component by propranolol. In the presence of atropine and propranolol, sustained sympathetic stimulation sometimes evoked a small, delayed pressor response which was blocked by phentolamine. Under the same conditions, transmural stimulation produced a depressor response evidently due to non-adrenergic, non-cholinergic nerves. Spontaneous activity was observed in some preparations under normal conditions, but could always be evoked by hypoxia. Responses to sympathetic stimulation were reduced both by hypoxia and during periods of spontaneous activity. 8 The principal advantage of this preparation is that it permits both excitatory and inhibitory responses to be elicited by stimulation of vagal and sympathetic nerves separately in the isolated trachea in the absence of agonist and antagonist drugs.

Discrimination by dipyridamole of two types of responses to adenosine and ATP of the carbachol-induced contraction of guinea-pig trachea

Two types of effect of purines on the carbachol-induced contraction of the guinea-pig trachea in the presence or absence of glucose were evaluated using dipyridamole, and adenosine uptake inhibitor. ATP exerted a greater relaxant effect on the carbachol (10(-6) M)-induced contraction than did adenosine in concentrations ranging from 10(-4) to 10(-3) M. These effects of purines were enhanced by pretreatment with dipyridamole (10(-5) M) but were virtually unaffected by the purine receptor antagonists theophylline (10(-5) M) and quinidine (10(-5) M). The contraction evoked by carbachol was gradually reduced and was almost abolished 4-5 h after exposure to the glucose-free solution. This reduction was almost eliminated by readmitting glucose, and was restored to a certain degree by introducing ATP or adenosine. ATP was less effective than adenosine. This effect of purines but not that of glucose, was markedly diminished by pretreatment with dipyridamole (3 X 10(-6) M to 10(-5) M). The restoration of the carbachol-induced contraction following introduction of purines or glucose was completely antagonized by atropine (10(-6) M). These results suggest that the extracellular effects of adenosine and ATP on the carbachol-induced contraction in normal solution are potentiated by dipyridamole, whereas the possible intracellular effects of these purines in the glucose-free solution are inhibited by this adenosine uptake inhibitor.

Mechanism of beta-adrenergic relaxation of smooth muscle

The mechanism of beta-adrenergic relaxation was investigated in isolated smooth muscle cells. Beta-adrenergic agents stimulate cyclic AMP-dependent phosphorylation, enhance Na+/K+ transport and induce relaxation. The stimulation of Na+/K+ transport is obligatory for relaxation, and we suggest that this stimulation induces relaxation through enhanced Na+/Ca2+ exchange.