Cell volume as a factor influencing electrical and mechanical activity of vascular smooth muscle

Osmolarity-induced renin secretion from kidneys: evidence for readily releasable renin pools

Our study aimed to characterize the influence of changes in extracellular osmolarity on renin secretion from the whole kidney. For this purpose, the osmolarity of the perfusion medium of isolated rat or mouse kidneys was either decreased by lowering the NaCl concentration by 20% or was increased up to 133% by the addition of various salts or sugars. It turned out that changes in osmolarity led to instantaneous transient changes followed by a plateau of renin secretion, in that increases in osmolarity stimulated renin secretion, whereas decreases attenuated renin secretion. The peak amplitude of changes in renin secretion was related to steady-state renin secretion rates before the osmotic challenge but was independent of the maneuver used to modulate steady-state renin secretion. Osmolarity-induced changes in renin secretion were more related to relative rather than to absolute changes in osmolarity and were not dependent on the formation of nitric oxide or of prostanoids and did not require Na-K-2Cl cotransport function or swelling-activated chloride channels. Moreover, we obtained evidence that the pool of renin secretion excitable by hyperosmolarity is exhaustible and that its complete refilling takes at least 2 min. The observed behavior of renin secretion fits the concept about exocytosis proposing the existence of different pools of committed secretory vesicles, which have not yet undergone the final modification for initiation of exocytosis. Probably, a pool of readily releasable vesicles determines steady-state secretion rates from kidneys.

Hyperosmolality dilates rat skeletal muscle arterioles: role of endothelial K(ATP) channels and daily exercise

The purpose of this study was to investigate the mechanism underlying arteriolar responses to hyperosmolality and to determine the effects of daily exercise on this response. Dilator responses were measured in isolated, cannulated, and pressurized skeletal muscle arterioles. Osmolality was increased from approximately 290 to 330 mosmol/kgH(2)O by adding glucose, sucrose, or mannitol to the superfusion solution. All three compounds elicited similar changes in vessel diameter, suggesting that this response was due to changes in osmolality. Responses to glucose were abolished by endothelium removal but were not altered in endothelium-intact vessels by superfusion with the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine or the cyclooxygenase inhibitor indomethacin. In endothelium-intact arterioles, responses to glucose superfusion with the ATP-sensitive potassium (K(ATP)) channel inhibitor glibenclamide; however, intraluminal perfusion with glibenclamide nearly abolished the responses to glucose and mannitol. Intraluminal administration of glucose elicited a significantly greater dilation than extraluminal glucose. The response to intraluminal glucose was also inhibited by intraluminal glibenclamide. Four weeks of daily exercise did not significantly alter the responses to hyperosmolality in gracilis or soleus muscle arterioles. These data demonstrate that physiological increases in intraluminal osmolality dilate rat skeletal muscle arterioles via activation of endothelial K(ATP) channels; however, this endothelium-dependent response is not augmented by daily exercise.

Influence of increase in osmotic pressure with sucrose on relaxation and cyclonucleotides levels in isolated rat aorta

The influence of increases in osmolarity by addition of sucrose were investigated on relaxation and changes in adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels in isolated rat aortic rings. Isoprenaline-mediated relaxations were attenuated in hypertonic (341+/-0.4 mOsmol) (mean+/-S.E.M.) solution. The concentration-response curve to isoprenaline was displaced to the right. The EC(50) (0.16+/-0.05 vs. 1.14+/-0.5 microM) significantly (n=6; P<0.05) increased without any changes to the maximum response. Hypertonic solution also attenuated methacholine-mediated relaxations resulting in a significant increase in the EC(50) (0. 28+/-0.04 vs. 0.52+/-0.04 microM) and reduced the maximal response (73+/-5% vs. 51+/-8%). In contrast, an increase in tonicity did not have any influence on sodium nitroprusside, forskolin or pinacidil concentration-response curves. Hypertonic solution also did not affect either basal cyclic AMP or cyclic GMP production. In addition, an increase in osmolarity did not affect isoprenaline-stimulated increases in the levels of cyclic AMP. However, an increase in the tonicity of Krebs solution significantly inhibited methacholine-stimulated (58%-34%) accumulation of cyclic GMP. The present data indicated that an increase in the tonicity of Krebs solution impaired endothelium-dependent relaxation and the associated increase in cyclic GMP production without affecting basal levels of this nucleotide. The inhibitory effects of high osmolarity on beta-adrenoceptor-mediated relaxation did not appear to be due to a reduction in cyclic AMP generation, or the result of inhibition of pinacidil-sensitive K(ATP)(+) channels. Moreover, an increase in the tonicity of Krebs solution did not influence relaxation induced by direct activation of adenylate cyclase or guanylate cyclase by forskolin and sodium nitroprusside, respectively.

Investigations into the mechanisms of coronary vasodilation by contrast media in dogs

RATIONALE AND OBJECTIVES

The study was performed to clarify the mechanisms underlying contrast-induced coronary vasodilation.

METHODS

The left anterior descending coronary artery of 14 open-chest dogs was perfused at constant pressure. Coronary blood flow (CBF) was measured electromagnetically and used to calculate myocardial oxygen consumption (MVO2). Responses were evaluated during intracoronary infusions (2 mL/ minute) of the ionic contrast medium, Hypaque-76, and the nonionic contrast medium, Isovue-370, and compared with those caused by hypertonic saline solutions with comparable osmolarities. Studies also were conducted using Isovist-300, which is a new nonionic and iso-osmolar contrast medium.

RESULTS

Hypaque-76 and Isovue-370 caused initial peak increases in CBF (reflecting decreases in coronary vascular resistance), which waned rapidly to achieve more modest steady-state increases within 2 to 3 minutes. Both the peak and steady-state increases in CBF were greater during Hypaque-76 than during Isovue-370. The increases in CBF caused by the contrast medium were greater than those caused by the corresponding saline solution. Neither Hypaque-76 nor Isovue-370 changed MVO2-Isovist-300 had no effect on CBF or MVO2.

CONCLUSIONS

The coronary vasodilation by contrast media is the result of a direct vasorelaxing effect rather than secondary to a metabolic mechanism. Hyperosmolarity can account only in part for contrast-induced coronary vasodilation.

Ca2+ entry and vasoconstriction during osmotic swelling of vascular smooth muscle cells

Exposure of aortic strips from guinea-pigs to hypotonic extracellular fluid is followed by marked vasoconstriction, which is inhibited by D-600 (3 microM), a blocker of voltage-sensitive Ca2+ channels. Conventional electrophysiology, patch-clamp studies, pH determination with 2',7' bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) and Ca2+ measurements with Fura-2 have been performed on smooth muscle cells cultured either from rat or human aorta to further elucidate the underlying mechanisms. Exposure of the cells to a 25% hypotonic extracellular fluid leads to a rapid and fully reversible depolarization, paralleled by an increase of the selectivity and conductance of the cell membrane to Cl-, an acidification of the cytoplasm and an increase of intracellular Ca2+ concentration ([Ca2+]i). The latter is inhibited by the Ca2+ channel blocker D-600 (1-3 microM). It is concluded that osmotic cell swelling leads to the activation of an anion channel. The subsequent depolarization of the cell membrane activates voltage-sensitive Ca2+ channels which increases [Ca2+]i, thus stimulating the contraction of vascular smooth muscle cells.

Effects of iodinated contrast media on peripheral blood flow

All types of clinically employed iodinated roentgen contrast media (CM) cause vasodilatation after i.a. and i.v. administration, regardless of precise molecular structure. It is now apparent, however, that at iodine concentrations which provide equivalent angiographic contrast, this is significantly less with newer hexa-iodinated dimers, such as iodixanol and iotrolan, than older generations of compounds. The cellular mechanisms that underly the vasodilator effects of CM still remain to be fully elucidated but may include a) effects attributable to hyperosmolality; b) stimulation of the release of endogenous vasoactive mediators; and c) direct relaxant effects upon vascular smooth muscle. This review will discuss the possible contributions of these mechanisms to the vasodilatation observed in the clinical situation.

Membrane stretch evoked by cell swelling increases contractile activity in vascular smooth muscle through dihydropyridine-sensitive pathways

Effects of the dihydropyridine calcium antagonist felodipine and of calcium reduction were studied on osmotically induced contractile responses in the vascular smooth muscle of the rat isolated portal vein. Previous studies have shown that changes in osmolarity that cause cell swelling are accompanied by increased contractile activity in this smooth muscle (Johansson & Jonsson 1968). A transient enhancement of the contractile activity developed in the portal vein on return to standard Krebs solution after exposure to 60 mM urea. This osmotic response was dependent on extracellular Ca2+ (abolished in Ca2+ free solution +0.1 mM EGTA) and was reduced in proportion to the decrease in spontaneous phasic contractile activity when Ca2+ was lowered from the standard 2.5 mM concentration. Felodipine, 3 nM, reduced the spontaneous activity to approximately 50% but showed an even more pronounced inhibitory effect on the osmotic responses which were reduced to less than 20% of control. Other calcium antagonists such as verapamil, 60 nM and diltiazem, 300 nM, were also more effective in inhibiting the osmotic responses than the spontaneous activity. In contrast, the K+ channel opener, pinacidil, 100-200 nM, reduced the spontaneous activity to 50% but had only minor inhibitory effect on the osmotic responses, about 75% still persisting. It is suggested that stretch of the cell membrane in response to variations in osmolarity induces contractile activity in vascular smooth muscle by mechano-electrical coupling involving dihydropyridine-sensitive pathways.

Unusual effects of SCN and lyotropic anions on contractility of vascular smooth muscle from female rats

Replacement of extracellular chloride ions by thiocyanate anions (SCN-) followed by washout in normal chloride-containing solution produced contractions in isolated rat aortas and portal veins of female rats followed by slow relaxation; these contractions consisted of fast and slow phases. These SCN(-)-induced biphasic contractions were also noted in rat aortas precontracted by 80 mM KCl and 100 microM noradrenaline. No differences were noted between isolated aortic precontracted by 80 mM KCl and 100 microM noradrenaline. No differences were noted between isolated aortic strips versus intact ring preparations. The SCN(-)-induced contractions in both the aorta and portal vein were inhibited markedly by denervation with 6-hydroxydopamine. Use of prazosin, rauwolscine, propranolol, atropine, methysergide, diphenydramine, indomethacin or procaine (10(-3) M) failed to alter the SCN(-)-induced responses. However, use of phentolamine at 10(-5) M, but not at lower concentrations of the drug, resulted in complete inhibition of SCN(-)-induced contractions. Treatment of the vascular tissues with EGTA (5 mM) or incubation in Ca(2+)-free media abolished the SCN(-)-induced contractile responses. Treatment with verapamil (10(-6) M) or washing in Ca(2+)-free Krebs Ringer solution after incubation with SCN(-)-Krebs Ringer selectively inhibited the slow phases of the aortic contractions. Replacement of SCN- anions with other foreign monovalent anions or with sucrose modified the amplitude of the SCN(-)-induced contractions. These foreign anions seemed to follow a relative order of potency similar to that for a lyotropic series of anions, where acetate greater than isethionate greater than chloride greater than bromide greater than nitrate greater than iodide ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional and species differences in vascular reactivity to extracellular potassium

In-vitro vasoreactivity to extracellular potassium (Ko+) was tested in isolated human pial and mesenteric arteries as well as basilar and mesenteric arteries from rabbits and rats. Contractions were induced by stepwise increases in [K+]o and were measured isometrically with a force-displacement transducer, in small-volume organ baths. Significant differences between species as well as between regions were found. The threshold of [K+]o for eliciting contraction in human cerebral arteries in hyperosmotic solutions was 10 mM, in rabbit cerebral arteries 17 mM and in rat cerebral arteries 27 mM. The threshold concentration for contraction in mesenteric arteries was significantly higher compared to cerebral arteries in humans and rabbits, but lower in rats: 20 mM in humans, 26 mM in rabbits and 25 mM in rats. In all species the contractile amplitudes were significantly higher in both cerebral and mesenteric arteries when [K+]o was increased under isotonic conditions in the buffer solution than when hyperosomolality was created. This difference increased with increasing hyperosmolality. In hyperosmotic solutions, the EC50 for [K+]o was lower in cerebral and mesenteric arteries from man than in vessels from rabbit and rat. When the solutions were isotonic, this pattern was seen only in mesenteric arteries. It is concluded that significant species and regional differences in vascular responses to [K+]o exist. Considering that [K+]o is increased in cerebral ischaemia, the observed significantly lower threshold for K+-induced contractions in human cerebral arteries may be of importance, especially in human cerebral ischaemic events.

Effect of urea, glycerol, and mannitol on basilar and labyrinthine arteries

We studied the effect of urea, glycerol, and mannitol on basilar and labyrinthine arteries of dogs in vitro by use of tension recording methods. When the tissue was exposed to urea or glycerol, temporary relaxation of smooth muscles was initially observed, but was followed by gradually increasing contraction. With mannitol, relaxation of the smooth muscles remained stable as long as mannitol existed in the bathing solution. When the specimens were washed with normal Krebs solution after exposure to any of these three substances, large phasic contractions of the smooth muscles were observed. While only theoretical, we propose a possible explanation for the different effects of these three drugs in the improvement of hearing loss that results from Meniere's disease.

Relaxation induced by KCl, NaCl and sucrose in rabbit coronary arteries

Contractions induced in isolated ring segments of rabbit coronary arteries by 2-(2-aminoethyl)-pyridine (AEP) were transiently relaxed when KCl, NaCl or sucrose were added to the bathing solution without osmotic correction. When these solutes were added with osmolarity changes minimized by reducing the concentration of another constituent of the medium the relaxations were reduced or abolished. AEP contraction was associated with depolarization and solute-induced relaxation with a lessening of the depolarization. Ouabain depolarized the vessel and diminished both the relaxation and repolarization induced by hyperosmolar addition of NaCl and sucrose to AEP-contracted segments. Sucrose and NaCl also relaxed potassium-induced contractions and their effect was greater at [K]o 25 mM than at [K]o 60 mM. Hyperosmolar relaxation still occurred but was significantly smaller in vessels subjected to prolonged sodium pump inhibition produced by cold storage followed by re-warming in K-free or ouabain-containing solution. It is concluded that hyperosmolar relaxation is mainly due to hyperpolarization and is influenced by the level of membrane potential. The inhibition of hyperosmolar relaxation by ouabain may be due to its depolarizing action.

The effects of hyperosmolar solutions on cerebral arterial smooth muscle

The present study was conducted to clarify the effect of hyperosmolar solutions on the constrictor responses of cerebral arteries to vasoactive agents, in vitro. The canine basilar arteries under resting tension were slightly relaxed with both mannitol (0.5, 1 and 2%) and sucrose (1, 2, and 4%). Constrictor responses of canine basilar arteries to 40 mM K+, 10(-7) M serotonin or 10(-6) M prostaglandin F2 alpha (PGF2 alpha) were markedly suppressed by pretreatment with either mannitol or sucrose. The rate of suppression correlated well to osmolarity changes in the Kreb's solution. When the specimens were incubated in Ca++-free medium, 10(-6) M PGF2 alpha elicited small contractions. Addition of 1 mM Ca++ to the bath promptly elicited larger contractions. The large contractions in response to the influx of extracellular Ca++ were markedly suppressed by pretreatment with mannitol or sucrose, while the small contractions induced by intracellular Ca++ were not inhibited. In addition, the contractions induced by the addition of Ca++ to the specimens depolarized with 80 mM K+ in Ca++-free medium were dose-dependently inhibited with either mannitol or sucrose, while the caffeine-induced contractions in Ca++-free medium were not altered by mannitol. These results suggest that hyperosmolar solutions produce non-specific vasodilation of cerebral arteries by inhibiting the influx of external Ca++ rather than the release of intracellularly stored Ca++. This direct vasodilatory effect may account in part for the transient increase of cerebral blood flow following administration of hyperosmolar mannitol.

Hyperosmolality and transcapillary fluid and protein movement in skeletal muscle

The effect of hyperosmolal glucose infusion on the transcapillary transport of fluid and protein was examined in the isolated gracilis muscle of the dog. Volumes of 18, 36, and 72 microliter/min of 30% glucose were infused intraarterially for 30 min into blood flow held constant at 3-4 ml/min X 100 g, thereby increasing plasma osmolality from 21 to 126 mOsm/liter. Transcapillary fluid movement (Jv) was assessed plethysmographically and protein transport (Js) by direct monitoring of the rate of increase of tissue 125I-albumin radioactivity. Hyperosmolal perfusion reduced vascular resistance. Initially tissue volume declined and then while the infusion continued, reversed direction and rose, occasionally exceeding control volume within the 30-min infusion period. Js also declined transiently, then increased during infusion. The reversal of Js led the reversal of Jv, therefore the reversal of Jv during hyperosmolal infusion was attributed to development of a sufficient change in the oncotic gradient due to the increased Js. These changes were significantly dose dependent. Stopping the hyperosmolal infusion resulted in a marked increase in Jv at all dose levels and, except for the low dose, a maintained rate of increase of Js. Thus early during hyperosmolal exposure protein transport was enhanced by either diffusion or vesicular transport. During recovery, the maintained increase in Js was probably due to increased convection.

Regional cerebral blood flow decreases during hyperglycemia

The presence of hyperglycemia before brain ischemia increases stroke-related morbidity and mortality in experimental animals and humans. However, little is known of the effect of hyperglycemia on regional cerebral blood flow (rCBF). Acute hyperglycemia was induced in awake but restrained rats by intraperitoneal injection of 50% D-glucose. Regional flow was determined using [14C]iodoantipyrine and quantitative autoradiography. Elevation of plasma glucose from 11 to 39 mM was associated with a 24% reduction in rCBF when compared with controls that received normal saline. Intraperitoneal D-mannitol produced an elevation of plasma osmolality equivalent to that observed with glucose. However, rCBF was only reduced by 10%. Hyperglycemia appears to produce a global decrease in rCBF in awake rats that cannot be completely explained by the attendant increase in plasma osmolality. If a similar influence is present during brain ischemia, hyperglycemia could extend areas of critical flow limitation.

Effect of sodium concentration and of atropine on the contractile response of the guinea-pig ileum to potassium ions

The possibility that the guinea-pig ileum's contractile response to K+-rich solutions is partly mediated by acetylcholine release from the intramural nervous tissue was examined by studying the inhibition of that response by atropine at different values of [Na+]o. In a medium in which the NaCl was replaced by iso-osmotic glucose, the response to high [K+]o was not greatly affected, while the responses to acetylcholine and to other agonists were significantly reduced. In control medium (149 mM Na+), atropine (10(-7) M) partly inhibited the responses to K+-rich solutions and to agonists such as histamine, 5-hydroxytryptamine and bradykinin. When [Na+]o was reduced to 12 mM, by iso-osmotic substitution of glucose for NaCl, the response to high K+ was no longer inhibited by atropine, which still partly inhibited the contractions elicited by the three agonists and totally blocked the response of acetylcholine. It is proposed that atropine's inhibition of the response to high K+ and to agonists is not due to its specific anti-muscarinic effect, but to an unspecific action, which in the case of the agonists is independent of [Na+]o. In addition, the inhibition of the response to high K+ would results from a different Na+-dependent mechanisms, possibly involving the stimulation of the Na-K pump by atropine. This is supported by the observation that this drug partly relaxed ileum preparations that were contracted by ouabain.

Pathogenesis of rheumatoid arthritis: a vascular hypothesis

Past research into the pathogenesis of RA has generally concerned itself with established inflammation. The present review summarizes alterations in microvascular anatomy and function which occur during the hypoxic state, in various experimental and disease conditions. It further shows that tissue hypoxia is a common finding in RA and that the microvascular alterations of RA are similar to those produced by experimental hypoxia. The available data suggest that microcirculatory compromise, concomitant with an increase in metabolic needs of synovial tissue, may initiate tissue injury via anoxia and acidosis, resulting in hydrolytic enzyme release, increased vascular permeability and acceleration of inflammatory processes. It is further believed that the microcirculatory abnormality may be generalized, accounting for the systemic manifestations often seen in RA. Factors effecting arteriolar blood flow obstruction are reviewed to identify areas for future investigation in RA and other disorders involving microvasculopathy. The multitude of longknown and newly recognized factors predisposing to vasospasm and vasodilatation have been outlined as a guide to possible mechanisms which may be operative in RA. An attempt has been made to gather and synthesize the available data in the hope that it may stimulate other investigators to pursue more definitive research into specific areas which may show early microvascular abnormalities in the pathophysiology of RA. Identification of factors operative early in the pathogenesis of RA, before it becomes self-perpetuating, may well be a step in the direction of preventing the ravages of this disease, or providing insight to more effective control.

Hyperosmolality, acetate, and lactate: dilatory factors during peritoneal dialysis

Factors that alter peritoneal blood flow may influence the clearance of solutes during peritoneal dialysis. Arteriolar vasodilation, for instance, could increase the delivery of solutes to the capillaries and venules leading to an increase in solute transport into the peritoneal cavity. This study was designed to identify the vasoactive effects of several major components of McGaw and Dianeal peritoneal dialysis solutions to understand how the composition of these solutions may alter in vivo blood flow in the peritoneum. Because the major differences between these solutions and Krebs solution are a high osmolality, a high dextrose concentrations, and an acetate or lactate buffer system, we investigated the effects of these components. Rats were anesthetized with the combination of urethane and chloralose. The cremaster muscles, with the nerve and blood supplies from the rat still intact, was placed in a specially designed tissue chamber that was filled with Krebs solutions. A port permitted microscopic observations of the blood vessels. In vivo television microscopy observations was used to quantitate changes in small arteriole diameters induced by changes in the composition of the solution bathing the cremaster or by the addition of nitroprusside. Hyperosmolality produced by the addition of dextrose, sucrose, or sodium chloride to the Krebs solution induced a submaximal dilation of the small arterioles of the cremaster. The rate of dilation differed depending on the substance used to increase osmolality. A normal osmolality acetate (74 mM) or lactate (45 mM) solution produced a slow, submaximal dilation of the cremaster arterioles. Hyperosmolar acetate (37 or 74 mM) or lactate (45 mM) solutions produced a rapid, maximal dilation of these vessels. Because the rate of dilation and maximal effect produced by the commercial dialysis solutions were similar to these same parameters produced by the high-osmolality acetate or lactate solutions, the dilatory effects of McGaw and Dianeal solutions appear to be due to the combinations of high osmolality and the buffer anion acetate or lactate.

Quantitative aspects of electrical and mechanical responses to anisosmolar solutions in the smooth muscle of the rat portal vein

The electrical and mechanical activity of the rat portal vein were studied quantitatively under prolonged exposure to solutions in which osmolality was varied by changes in sucrose content. Reducing osmolality by 15 or 30 mosm/kg below the control value of 290 mosm/kg caused enhanced electrical and mechanical activity whereas hyperosmolality up to 390 mosm/kg led to inhibition as demonstrated earlier. These responses faded under prolonged exposure. In hypoosmolality integrated mechanical activity returned to control after about 10-15 min while spike activity remained somewhat increased. Prolonged hyperosmolality was associated with return of spike discharge towards control frequency whereas the integrated contractile force reverted from initial inhibition to levels above control after some 10-15 min. Therefore, the integrated force per spike was increased by prolonged hyperosmolality and decreased by hypoosmolality. Variation in osmolality had similar effects on the amplitude of K+ contractures. The time course of the osmotic responses are discussed in relation to the dynamic effects of passive stretch and shortening in the portal vein. The relation that may exist between "intropic" effects of osmolality and the contractures obtained in strongly hypertonic media is considered.

Effects of increasing osmolality on rat ileal smooth muscle

The effects of changing bathing medium osmolality on tension generation in smooth muscle were studied on potassium-depolarized segments of rat ileum. Increasing the tonicity of the medium evokes a transient relaxation of the smooth muscle; restoration of isotonicity evokes a transient contraction of similar amplitude and time course. While immersed in hypertonic media of less than 1.3 times normal tonicity, the smooth muscle slowly increases its isometric tension; in media of nearly twice normal tonicity, the smooth muscle loses tension. All of these effects occur whether the medium is made hypertonic with added NaCl or sucrose.

The contractile response of smooth muscle to immersion in hypertonic solutions

1. Strips of bovine tracheal muscle and rabbit aorta produced sustained contractions on perfusion with Krebs solution made twice normal strength by addition of sucrose. The contractures were relaxed on return to normal Krebs solution. 2. Similar contractures were produced by tracheal muscle strips in Krebs solutions made twice normal strength by addition of galactose, glucose or NaCl whereas urea caused only a transient contraction. 3. In twice normal strength Krebs solution (sucrose added) the basal tension of rat portal vein and guinea-pig taenia coli was increased. Spontaneous mechanical activity was maintained, but the frequency of contractions was reduced. 4. The hypertonic contracture of bovine trachea in twice normal strength Krebs solution (sucrose added) was reduced by 15% by omission of Ca from the bathing fluid (0.1 mmol/l EGTA added). Severe Ca depletion, by prolonged washing in Ca-free Krebs with 12.5 mmol/l EGTA and Carbachol added, resulted in a 77% reduction in the hypertonic contracture. 5. In twice normal Krebs solution (sucrose added), the hypertonic contracture was partially relaxed by isoprenaline (4 x 10(-6) mol/l); the contractile response to carbachol was reduced; the contractile response to high-K Krebs solution was maintained. 6. Atropine (5 x 10(-7) mol/l) abolished the contractile response to carbachol, but had no effect on the hypertonic contracture. 7. It is suggested that the contraction of bovine tracheal strips in hypertonic solutions is mainly due to activation of the contractile myofilaments rather than simple cell shrinkage. Hypertonic solutions may also interfere with some steps in the excitation-contraction coupling sequence.

A fixed charge system as a formal model for the behaviour of smooth muscles and the heart

An extension of the theory of the "fixed charge" controlled "membrane oscillator" is used for simulation of a variety of excitability phenomena in nerves, presso-receptors, smooth muscle and the heart. A central issue is the prediction of water movements across the cell membrane resulting in "swelling" and "shrinkage". The presented "electrohydraulic excitability analog" (EHEA) deals particularly with the generation of action potentials, volume-pressure changes and contractility in response to externally applied mechanical stimuli, i.e. the EHEA is applied as a pressoreceptor analog. The agreement between the theoretical model results and some experimental physiological observations cited from the literature is quite satisfactory.

Urethane and contraction of vascular smooth muscle

1 In vitro studies were undertaken on rat aortic strips and portal vein segments in order to determine whether or not the anaesthetic, urethane, can exert direct actions on vascular smooth muscle. 2 Urethane was found to inhibit development of spontaneous mechanical activity. This action took place with a urethane concentration as little as one tenth of that found in anaesthetic plasma concentratios, i.e., 10(-3) M. 3 Urethane (10(-3 to 10(-1) M) dose-dependently attenuated contractions induced by adrenaline, angiotensin and KCl. These inhibitory actions were observed with urethane added either before or after the induced contractions. 4 Ca2+-induced contractions of K+-depolarized aortae and portal veins were also attenuated, dose-dependently, by urethane. 5 All of these inhibitory effects were completely, and almost immediately, reversed upon washing out the anaesthetic from the organ baths. 6 A variety of pharmacological antagonists failed to mimic or affect the inhibitory effects induced by urethane. 7 These data suggest that plasma concentrations of urethane commonly associated with induction of surgical anaesthesia can induce, directly, relaxation of vascular muscle.

The effects of blood osmolality changes on cat carotid body chemoreceptors in vivo

The possibility that carotid chemoreceptors respond to changes in plasma osmolality was investigated in the cat, perfusing the carotid artery with blood made hyper- or hypo-osmotic and recording chemoreceptor activity from carotid nerve fibers. Blood made hyperosmotic with sucrose or NaCl reduced the chemoreceptor discharge, while hypoosmotic blood increased chemoreceptor activity. The minimal osmolality variation necessary to obtain a detectable frequency change was 3--8% of the control. Frequency changes of 30% of the control were obtained with a 20% variation in osmolality. The frequency variations produced by the osmotic changes lasted as long as the infusion was maintained (up to 15 min). In some instances a rebound was observed when iso-osmotic saline was perfused again. A transient change in frequency and a clear rebound were obtained when blood made hypersomotic with glycerol was perfused. These effects probably reflect a rapid change in intracellular osmolality due to the free passage of glycerol across cellular membranes. The modifications in chemoreceptor activity consecutive to osmolality variations are the opposite of those observed in isolated and superfused carotid bodies. As it is known that osmolality values affect the smooth muscle of the blood vessels, we conclude that our results are mainly produced by changes in carotid body blood flow due to a direct effect of hyper- and hypo-osmotic solutions on vascular muscle tone. Chemoreceptor excitation during a decrease in blood osmolality may contribute reflexly to the increased vascular resistance observed during acute osmolality reductions in man.

Hyperosmolality and pancreatic blood flow

In the cat pancreas, close intra-arterial infusions of hypertonic xylose, glucose and sucrose solutions caused a marked vasodilatation but no secretion. The magnitude of the vasodilator response correlated with the degree of induced glandular hyperosmolality but not with the substance per se. Stimulation of the vagus nerve as well as infusion of secretin and/or CCK evoked a pronounced secretion and a small to moderate increase in pancreatic blood flow, but did not significantly change the plasma osmolality in the venous effluent from the gland. Bradykinin, papaverine and isoprenaline caused large increments of pancreatic blood flow; none of these drugs evoked secretion nor did they change the venous osmolality. It is concluded that blood-bone hyperosmolality is a strong stimulus for pancreatic vasodilatation, but the functional hyperaemia of this gland is due to factors other than regional tissue hyperosmolality.

Some pharmacological properties of the cremaster muscle of the guinea-pig

1 The tension developed in the guinea-pig cremaster was recorded during spontaneous activity and electrical stimulation. Spontaneous rhythmic contraction was observed in many preparations, particularly in the tip of the cremaster. These contractions were very slow, lasting about 20 s and occuring at about 2 min intervals, but different preparations varied greatly. Twitches produced by electrical stimulation were similar to those in other skeletal muscles, being reduced by (+)-tubocurarine and abolished by tetrodotoxin. 2 A slow contraction could be initiated by electrical stimulation using a pulse longer than 10 ms in spontaneously active preparations and in some quiescent preparations. Spontaneous and evoked slow contractions were not suppressed by tetrodotoxin, but were selectively abolished by verapamil. Histamine increased the basal tension and generated spontaneous contractions in quiescent preparations. Hypertonic solutions also had excitatory effects. 3 Contractions produced by acetylcholine and carbachol were blocked by atropine. Those produced by adrenaline and noradrenaline were stronger than those due to histamine, acetylcholine and carbachol and were abolished by an alpha-adrenoceptor blocking agent, phentolamine. In the preparations in which the slow contraction was not observed, histamine, acetylcholine or adrenaline had very little effect. 4 It is suggested that the cremaster muscle consists of striated muscle together with some smooth muscle having properties similar to that in the vas deferens.

Circulatory effects evoded by 'physiological' increases of arterial osmolality

The effects of moderate arterial hyperosmolality (+20 mOsm/kg H2O), produced by short term intravenous hypertonic infusion, on vascular resistance in skin, skeletal muscle, intestine, and kidney were analyzed in the anesthetized cat. Vascular resistance decreased in all four regions in response to the hypertonicity both before and after regional sympathectomy and the effects were not significantly altered by beta-adreno-ceptor blockade. Arterial blood pressure rose during the hypertonic infusion despite the decreased vascular resistance and an unchanged heart rate, indicating an increased stroke volume and cardiac output. Similar increases of arterial osmolality are known to occur in heavy exercise and in hemorrhage. The present results may therefore suggest that blood borne hyperosmolality is a factor which can contribute to the overall cardiovascular adjustments in these situations.

Evidence in support of hypoxia but against high potassium and hyperosmolarity as possible mediators of sustained vasodilation in rabbit cardiac and skeletal muscle

The vasoactive properties of hypoxia, elevated extracellular potassium concentration ([K] o ), and hyperosmolarity were studied in helical strip preparations of small coronary and deep femoral arteries (260-700 µm, o.d.) equilibrated in a physiological salt solution with an oxygen tension (Po 2 ) of 100 mm Hg, [K] o of 3.18 mM, and an osmolarity of 304 milliosmoles/liter. Increasing [K] o (2-6 mM) or osmolarity (30-50 milliosmoles/liter) produced relaxation of resting tension in 50% of the coronary strips but had no effect on resting tension in deep femoral strips. Sustained increments in [K] o or osmolarity produced concentration-dependent, transient relaxation of aganist-induced contractile tension in both coronary and deep femoral arterial strips: a potassium increment of 4 mM produced 40% relaxation with 100% recovery within 5-6 minutes, an osmotic increment of 30 milliosmoles/liter caused 20-40% relaxation with 100% recovery within 15-60 minutes, and simultaneous potassium (4 mM) and osmotic (30 milliosmoles/liter) increments produced 85-95% relaxation with 100% recovery within 10-15 minutes. Decreasing Po 2 from 100 mm Hg to 10 mm Hg resulted in a sustained 35-40% fall in agonist-induced contractile tension. Although a nonspecific additive interaction was observed during a simultaneous change to high [K] o , hyperosmolarity, and hypoxia, for any given level of vascular tone hypoxia had little or no effect on the degree or the duration of the tension relaxation-recovery sequence produced by elevated [K] o , hyperosmolarity, or both. Therefore, it is proposed that hypoxia is the only one of these three factors that, by a direct interaction with vascular smooth muscle cells, can contribute to sustained vasodilation of small arteries in rabbit cardiac or skeletal muscle.

Dependency of pial arterial and arteriolar diameter on perivascular osmolarity in the cat. A microapplication study

The effect of perivascular osmolarity on the diameter of pial arteries was studied in cats by the microapplication technique. Between 251 and 360 mosmoles/liter, concentration-response curves were obtained for single vessels. Constriction occurred when perivascular osmolarity was decreased below 317 mosmoles/liter, and dilation occurred at osmolarities above this value. The effect was the same whether the osmolarity was changed by addition of mannitol or NaCl. Reduction of sodium concentration from 156 to 133 mEq/liter at constant osmolarity did not affect arteriolar diameter, but greater reductions in sodium concentration induced vasoconstriction. The results indicate that the resistance of pial arteries can be influenced by local changes in perivascular osmolarity.

Influence of sodium and potassium content on arterial responsiveness

Vasoconstrictor responses to norepinephrine were determined during constant flow perfusion with warm Krebs solution in dog mesenteric arteries. The arteries were then stored 20-24 hours at 3°C in one of the following isosmotic solutions: Krebs, complex K (KC1, CaCl 2 , KHCO 3 and glucose), NaCl, KCl, sucrose, or mannitol. After storage, arteries were again perfused with warm Krebs solution, and vasoconstrictor responses to norepinephrine compared to prestorage control responses. Tissue levels of Na + and K + were determined before cold storage and after 0-35 minutes of poststorage perfusion with warm Krebs solution. Storage of arteries in mannitol or sucrose solution reduced their Na + and K + content, and during poststorage perfusion with warm Krebs solution their responsiveness gradually improved as they rapidly (within 5 minutes) regained Na + and slowly regained K + . Arteries stored in NaCl solution were initially more responsive than those stored in electrolyte- free solutions during poststorage perfusion with warm Krebs solution, and their responsiveness also improved as they regained K + . Arteries cold-stored in K + containing media (KCl and complex K solutions) quickly regained Na + , and their responsiveness exceeded prestorage values with 11 minutes of poststorage perfusion with warm Krebs solution. Since these data indicate that artery responsiveness may be related to tissue levels of both Na + + and K + efforts were made to establish minimal levels of each ion required for "normal" responsiveness. The critical level of Na + appears to be approximately 30 µEq Na/g dry weight; the minimal effective K + content is apparently near 20 µEq K/g dry weight. If arteries contain at least minimal levels of K + and an excess (over 100 µEq/g) of Na + the magnitude of theft responses to vasoconstrictor stimuli is exaggerated

Blood hyperosmolality and pulmonary vascular resistance in the cat

Blood osmolality rises during general muscle exercise. The purpose of this work was to investigate the effect of similar slow, graded rises of blood osmolality on pulmonary vascular resistance. Hyperosmolar solutions of sodium chloride, mannitol, urea, glucose, thiourea and ethylene glycol were infused into the left lower lobe pulmonary arterial branch in the cat. Lobe blood flow, pulmonary arterial, left atrial and systemic arterial pressures were recorded. With the exception of ethylene glycol, all the test solutions caused dose-dependent, reversible reductions in lobe vascular resistance (LVR). Maximal reduction in LVR was 30% of initial value. A rise of blood osmolality of 25 milliosmols/liter, comparable with levels found during exercise, caused a 10% reduction in LVR. Hyperosmolar solutions of ethylene glycol, which has the highest rate of cell wall penetration of the test substances, caused no change in LVR. Bolus injections of 1M solutions of sodium chloride caused rapid transient rises in LVR. It appears that the resistance vessels of the lungs react to graded rises of blood osmolality in a way qualitatively similar to that seen in the resistance vessels of skeletal muscle.