Effects of hyperosmolarity on the volume of vascular smooth muscle cells and the relation between cell volume and muscle activity

Vasoactive Components of Dialysis Solution

Background

Conventional peritoneal dialysis (PD) solutions elicit vasodilation, which is implicated in the variable rate of solute transport during the dwell. The components causing such vasoactivity are still controversial. This study was conducted to define the vasoactive components of conventional and new PD solutions.

Methods

Three visceral peritoneal microvascular levels were visualized by intravital video microscopy of the terminal ileum of anesthetized rats. Anesthesia-free decerebrate conscious rats served as control. Microvascular diameter and blood flow by Doppler measurements were conducted after topical peritoneal exposure to 4 clinical PD solutions and 6 prepared solutions designed to isolate potential vasoactive components of the PD solution.

Results

All clinically available PD solutions produced a rapid and generalized vasodilation at all intestinal microvascular levels, regardless of the osmotic solute. The pattern and magnitude of this dilation was not affected by anesthesia but was determined by arteriolar size, the osmotic solute, and the solution's buffer anion system. The greatest dilation occurred in the small precapillary arterioles and was elicited by conventional PD solution and heat re-sterilized solution containing low glucose degradation products (GDPs). Hypertonic mannitol solutions produced a dilation that was approximately 50% less than the dilation obtained with glucose solutions with identical osmolarity and buffer. Increasing a solution's osmolarity did not produce a parallel increase in the magnitude of dilation, suggesting a nonlinear relationship between the two variables. Lactate dissolved in an isotonic solution was completely non-vasoactive unless the solution's H+concentration was increased. At low pH, isotonic lactate produced a rapid but transient vasodilation. This vascular reactivity was similar in magnitude and pattern to that obtained with the isotonic 7.5% icodextrin solution (Extraneal; Baxter Healthcare, Deerfield, Illinois, USA).

Conclusions

( 1 ) Hyperosmolarity is the major vasoactive component of PD solution. ( 2 ) Hyperosmolarity and active intracellular glucose uptake account together for approximately 75% of PD solution-induced dilation, whereas GDPs contribute to approximately 25%. ( 3 ) Lactate is vasoactive only at low pH (high [H+]). ( 4 ) The magnitude of PD solution-mediated vasodilation is partially dependent on the nature of the osmotic solute, the GDP contents, and the [H+], which determine the vasoactivity of the lactate-buffer anion system. Studies are required to define the molecular mechanisms of PD-induced vasodilation and to determine the vasoactive properties of these solutions after chronic infusion.

Calibrated brightfield-based imaging for measuring intracellular protein concentration

Intracellular protein concentration is an essential cell characteristic, which manifests itself through the refractive index. The latter can be measured from two or more mutually defocused brightfield images analyzed using the TIE (transport-of-intensity equation). In practice, however, TIE does not always achieve quantitatively accurate results on biological cells. Therefore, we have developed a calibration procedure that involves successive imaging of cells in solutions containing different amounts of added protein. This allows one to directly relate the output of TIE (T) to intracellular protein concentration C (g/L). The resultant relationship has a simple form: C ≈ 1.0(T/V), where V is the cell volume (μm³ ) and 1.0 is an empirical coefficient. We used calibrated TIE imaging to characterize the regulatory volume increase (RVI) in adherent HeLa cells placed in a hyperosmotic solution. We found that while no RVI occurs over the first 30-60 min, the protein concentration fully recovers after 20 h. Because interpretation of such long experiments may depend on whether protein concentration varies significantly throughout the cell cycle, we measured this parameter in three cell lines: HeLa, MDCK and DU145. Our data indicate that protein concentration remains relatively stable in these cells. © 2017 International Society for Advancement of Cytometry.

Hyperosmolarity Causes BKCa-Dependent Vasodilatations in Rat Skeletal Muscle Arteries

PURPOSE

The release of different metabolites during skeletal muscle contraction causes a pronounced increase in extracellular osmolarity (hyperosmolarity (HO)). HO has been considered as a possible mediator of the exercise hyperemia. In the present study, we investigated the vasodilatory effect of physiologically relevant increases in the extracellular osmolarity in isolated rat gluteal muscle arterioles. In addition, we analyzed the underlying mechanisms of the HO-induced vasodilatations.

METHODS

Rat gluteal arteries were isolated and mounted in an organ bath for isometric tension recording. After precontraction with norepinephrine, 20, 40, or 60 mmol x L(-1) sucrose, mannitol, or urea was added in control conditions, after removal of the endothelium or in the presence of inhibitors.

RESULTS

Application of sucrose or mannitol induced large and fast concentration-dependent vasodilatations (up to 46.15% with 60 mmol x L(-1) sucrose). Removal of the vascular endothelium had no effect on this relaxation. Inhibition of the Na+/K+ pumps with ouabain, the Kir IR channels with Ba2+ and the K ATP channels with glibenclamide did not alter the HO-induced relaxations. Incubation with the KCa channel blockers charybdotoxin and apamin significantly inhibited sucrose-induced vasodilatations. In addition, application of the specific BK Ca channel blocker iberiotoxin significantly decreased the HO-induced vasodilatations.

CONCLUSION

The present study shows that an increase in the extracellular osmolarity elicits strong, fast, and long-lasting relaxations of rat skeletal muscle arterioles, suggesting an important role both at the onset and during the steady-state phase of an exercise bout. Vascular smooth muscle BK Ca channels seem to play a crucial role in the HO-induced vasorelaxations.

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.

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.

Effects of metabolic inhibition on cytoplasmic calcium and contraction in smooth muscle of rat portal vein

Contractions in the rat portal vein, evoked by spontaneous action potentials or depolarizing high-K+ solution, are rapidly and reversibly inhibited by hypoxia or respiratory blockade. Intracellular free calcium ([Ca2+]i) was measured using Fura-2 to evaluate the effects of metabolic blockade on excitation-contraction coupling. Spontaneous contractions were associated with transient increases in [Ca2+]i. During exposure to cyanide (0.2-0.4 mM) or 2,4-dinitrophenol (30 microM) the duration and amplitude of the Ca2+ transients were decreased, leading to a decreased mean time integral of the individual [Ca2+]i transient, and corresponding decrease in the duration and amplitude of the contraction. Basal [Ca2+]i was increased in the presence of the metabolic inhibitors. High-K+ (40 mM) contractions caused a sustained increase in [Ca2+]i, which was not inhibited by exposure to cyanide, although the amplitude of the associated contraction was greatly reduced. Together with the earlier demonstration of decreased 20 kD myosin light chain phosphorylation under these conditions, this indicates that the activation of contraction is influenced by metabolism via the energy dependence of the light chain phosphorylation reaction. Thus at least three steps in the excitation-contraction sequence are influenced by inhibition of oxidative metabolism: membrane excitation, light chain phosphorylation, and the cross-bridge cycle. This provides mechanisms for a high degree of metabolic sensitivity of vascular tone, of importance for the adaptation of blood flow to tissue metabolic demands.

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.

Water drinking caused by intracerebroventricular infusions of hypertonic solutions in cattle

Infusions into the lateral cerebral ventricle of hypertonic solutions of NaCl, mannitol or sucrose all induced water drinking in cattle. However, infusion of hypertonic NaCl caused a significantly greater water drinking response than did the infusions of mannitol or sucrose, despite the fact that CSF osmolality increase was similar. In contrast, hypertonic solutions of NaCl or mannitol had similar dipsogenic effects when infused intravenously. The intracerebroventricular infusions of hypertonic NaCl or mannitol did not affect the intakes of food or Na solution. The results are consistent with the hypothesis that both cerebral osmoreceptors and Na sensors are involved in regulating thirst in cows.

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.

Regulation of renal blood flow by plasma chloride

Micropuncture studies have shown that glomerular filtration rate (GFR) falls in response to a rise in Na(+) or Cl(-) concentrations in the loop of Henle, whereas studies in isolated kidneys have shown that GFR falls in response to osmotic diuresis. To define the separate effects of an acute increase in plasma sodium (P(Na)), chloride (P(Cl)) or osmolality (P(osmol)), changes in renal blood flow (RBF) and GFR were measured during intrarenal infusions of hypertonic NaCl, NaHCO(3), Na acetate, dextrose, NH(4)Cl or NH(4)acetate to denervated kidneys. The infusions raised P(osmol) at the experimental kidney by 30-45 mosmol. RBF increased abruptly by 10-30% with all hypertonic infusions indicating that an acute increase in plasma tonicity causes renal vasodilatation. Renal vasodilatation persisted or increased further during infusion of dextrose, NaHCO(3) and Na acetate, but GFR was unchanged. In contrast, during infusion of the two Cl-containing solutions, vasodilatation was reversed after 1-5 min and RBF and GFR decreased (P < 0.01) below preinfusion levels. Prior salt depletion doubled the vasoconstriction seen with hypertonic NaCl infusions. Overall, changes in RBF were unrelated to changes in P(Na) or fractional Na or fluid reabsorption but correlated with changes in P(Cl) (r = -0.91) and fractional Cl(-) reabsorption (r = 0.94). The intrafemoral arterial infusion of the two Cl-containing solutions did not increase femoral vascular resistance. In conclusion, hyperchloremia produces a progressive renal vasoconstriction and fall in GFR that is independent of the renal nerves, is potentiated by prior salt depletion and is related to tubular Cl(-) reabsorption. Chloride-induced vasoconstriction appears specific for the renal vessels.

Regulation of renal blood flow by plasma chloride

Micropuncture studies have shown that glomerular filtration rate (GFR) falls in response to a rise in Na(+) or Cl(-) concentrations in the loop of Henle, whereas studies in isolated kidneys have shown that GFR falls in response to osmotic diuresis. To define the separate effects of an acute increase in plasma sodium (P(Na)), chloride (P(Cl)) or osmolality (P(osmol)), changes in renal blood flow (RBF) and GFR were measured during intrarenal infusions of hypertonic NaCl, NaHCO(3), Na acetate, dextrose, NH(4)Cl or NH(4)acetate to denervated kidneys. The infusions raised P(osmol) at the experimental kidney by 30-45 mosmol. RBF increased abruptly by 10-30% with all hypertonic infusions indicating that an acute increase in plasma tonicity causes renal vasodilatation. Renal vasodilatation persisted or increased further during infusion of dextrose, NaHCO(3) and Na acetate, but GFR was unchanged. In contrast, during infusion of the two Cl-containing solutions, vasodilatation was reversed after 1-5 min and RBF and GFR decreased (P < 0.01) below preinfusion levels. Prior salt depletion doubled the vasoconstriction seen with hypertonic NaCl infusions. Overall, changes in RBF were unrelated to changes in P(Na) or fractional Na or fluid reabsorption but correlated with changes in P(Cl) (r = -0.91) and fractional Cl(-) reabsorption (r = 0.94). The intrafemoral arterial infusion of the two Cl-containing solutions did not increase femoral vascular resistance. In conclusion, hyperchloremia produces a progressive renal vasoconstriction and fall in GFR that is independent of the renal nerves, is potentiated by prior salt depletion and is related to tubular Cl(-) reabsorption. Chloride-induced vasoconstriction appears specific for the renal vessels.

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.

Hemodynamic effects of rapid bolus hypertonic sodium bicarbonate

We found that rapid bolus administration of hypertonic sodium bicarbonate in the recommended clinical dose produced significant, though transient, increases in intracranial pressure and decreases in blood pressure in dogs. These effects, which are related to the high osmolarity of the drug (2,000 mOsm/L), are not found when it is administered slowly.

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.

Mechanical, electrical, and biochemical effects of hypoxia and substrate removal on spontaneously active vascular smooth muscle

Effects of hypoxia and glucose-free solution on the isolated rat portal vein were studied. Decrease of extracellular PO2 below 50 mm Hg caused graded inhibition of spontaneous mechanical activity; below 7 mm Hg, inhibition was complete in most preparations. Contracture force of depolarized portal vein was less sensitive to decreases in PO2. Responses to noradrenaline at all concentrations were markedly depressed at extreme hypoxia. Sucrose-gap experiments showed that hypoxia reduced the spontaneous electrical spike discharge. Mean tissue contents of PCr, ATP and glycogen (expressed as glucose) were 3.02, 2.47 and 5.07 micromol/g cell wt. in spontaneously active control muscles and 1.07, 1.65 and 1.83 after 20 min anoxia. Physiological variations in PO2 may influence myogenic activity of vascular smooth muscle largely through an action at the membrane level and this mechanism may participate in local blood flow control. Caculations indicated that the graded response to hypoxia in the present in vitro experiments was not due to diffusion limitation. Spontaneous mechanical activity was relatively well maintained even after prolonged exposure to glucose-free solution, whereas the responses to K+ and noradrenaline were markedly suppressed. Electrophysiological recordings during spontaneous activity indicated desynchronization and impaired conduction. PCr and ATP were maintained at control levels and glycogen reduced by 50 per cent after 2 h in glucose-free medium. Indications of the use of amino acids (glutamate) as substrate under these conditions were obtained.

The distribution of sodium in aortic walls from spontaneously hypertensive and normotensive rats

The contents of exchangeable sodium, bound sodium and total water and the extracellular space of thoracic aortas from normotensive and spontaneously hypertensive rats were measured. The aortas from the hypertensive rats contained more sodium than those from the normotensive animals while the total water content and extracellular space in the two groups were the same. The capacity to bind sodium in an osmotically inactive form was greater in the aortas from the hypertensives than in those from the normotensives. The difference in binding capacity was of the same order of magnitude as the difference in sodium content, indicating that the excess sodium in the thoracic aortas from the hypertensive rats was osmotically inactive and thus unable to cause water logging.

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.

Potassium accumulation in smooth muscle and associated ultrastructural changes

1. The water content, extracellular ((60)CoEDTA) space, ionic composition and ultrastructure of several mammalian smooth muscles were studied after incubation in solutions of varying ionic compositions and osmolarities.2. Substitution of KCl for NaCl resulted in an increase in cell water, K and Cl, accompanied by little change in total wet weight. This was due to a reduction in the extracellular space.3. Changes in extracellular osmolarity produced a wider range of cell volumes in high KCl solutions than in Krebs. The addition of 29-58 mM sucrose to high KCl prevented the swelling.4. Electron microscopy of smooth muscle swollen in high KCl solution revealed light (less electron opaque than normal) fibres of increased diameter, reduction in extracellular space, and nuclear swelling. The normal thick filament lattice was destroyed in swollen, osmium-fixed smooth muscles.5. The ultrastructural changes ascribed to swelling were absent in smooth muscles, (a) depolarized in high K(2)SO(4) solutions, (b) in high KCl solutions with 29-58 mM sucrose, and (c) returned to normal Krebs solution for recovery from swelling.6. Smooth muscles incubated in high KCl (swollen) and high K(2)SO(4) (unswollen) exhibited similar contractile responses, suggesting the filament lattice was intact until fixation, and that the contractile mechanism can operate over a relatively wide range of actin to myosin separations.7. Shrinkage of smooth muscles in high KCl solutions made hypertonic with the addition of 10% sucrose was accompanied by an aggregation of the thick filaments.8. The cell water of fixed taenia coli was reduced (a) by incubation in hypertonic solution followed by fixation in normal glutaraldehyde, or (b) by fixation of normal tissue in hypertonic glutaraldehyde. Osmotic responses during aldehyde fixation may be a source of artifact in the visualization of the normal filament lattice.

Evidence for uneven alpha-receptor distribution in the rat portal vein

The smooth muscle of the rat portal vein which supports propagation becomes supersensitive to exogenous l -norepinephrine after elimination of the vessel's intramural neuronal uptake mechanism. Such supersensitivity is generally associated with intimate neuromuscular connections. Since the adrenergic vasomotor fibers in the portal vein do not enter the muscle tissue, it has been proposed that muscle cells close to the nerve terminals must determine the response to norepinephrine. With the aim of elucidating the mechanism behind the prejunctional supersensitivity, the total uptake of labeled norepinephrine in the portal vein was analyzed and its distribution within the vessel wall was evaluated directly using isotopically labeled frozen sections. The total uptake of the portal vein, related to its endogenous norepinephrine content, was comparable with that in other vascular and nonvascular tissues. The 3 H-uptake profile of the vessel wall showed that the neurogenic uptake was confined to the narrow adrenergic plexus between the longitudinal and the circular muscle layers. The neuronal uptake mechanism therefore could only influence the concentration of exogenous norepinephrine at muscle cells close to the nerve terminals. These results support the hypothesis that the prejunctional supersenstivity in the rat portal vein indicates that the alpha receptors are located on muscle cells in the vicinity of the adrenergic nerve terminals.

The membrane properties of the smooth muscle of the guinea-pig portal vein in isotonic and hypertonic solutions

The membrane properties of the longitudinal smooth muscle of the guinea-pig portal vein were investigated under various experimental conditions.1. In isotonic Krebs solution, the membrane potential (-48.7 mV), the maximum rates of rise and fall of the spike (4.6 and 2.3 V/sec respectively), the space constant (0.61 mm), the conduction velocity of excitation (0.97 cm/sec) and the time constant of the foot of the propagated spike (18.4 msec) were measured.2. The various parameters of the muscle membrane in the isotonic solution were compared with those in the hypertonic solution prepared by the addition of solid sucrose (twice the normal tonicity).3. When the muscles were perfused with hypertonic solution, marked depolarization of the membrane and increased membrane resistance occurred. These were probably due to reduction of the K permeability, increased internal resistance of the muscle and shrinkage of the muscle fibre.4. The membrane potential in isotonic and hypertonic solutions was analysed into two components, i.e. the metabolic (electrogenic Na-pump) and the ionic (electrical diffusion potential) component in the various environmental conditions.(a) In isotonic and hypertonic solutions, the membrane was depolarized by lowering the temperature or by removal of K ion from the solutions. When the tissues were rewarmed or on readdition of K ion, the membrane was markedly hyperpolarized. These hyperpolarizations of the membrane were suppressed by treatment with ouabain (10(-5) g/ml.), by warming to only 20 degrees C and by K-free solution.(b) The relationships between the membrane potential and the [K](o) in isotonic Krebs, in the hypertonic (sucrose) Krebs, in the Na-free (Tris) Krebs and in the Cl-deficient (C(6)H(5)SO(3)) Krebs were observed. The maximum slopes of the membrane depolarization against tenfold changes of [K](o) were much lower than that expected if it behaved like a K electrode.(c) In Na-free (Tris) solution, the membrane was not depolarized in isotonic condition but it was depolarized in hypertonic condition.5. The low membrane potential in hypertonic solution (-37 mV) compared with isotonic solution (-49 mV) was thought to be mainly due to suppression of K permeability of the membrane and not due to suppression of the metabolic component. The electrogenic Na-pump and the membrane potential of the portal vein was discussed in relation to other excitable cell membranes.