Permeability characteristics of vascular smooth muscle cells as revealed by their osmotic responses to non-electrolytes

Erythritol: an interpretive summary of biochemical, metabolic, toxicological and clinical data

A critical and comprehensive review of the safety information on erythritol was undertaken. Numerous toxicity and metabolic studies have been conducted on erythritol in rats, mice and dogs. The toxicity studies consist of long-term feeding studies conducted to determine carcinogenic potential, intravenous and oral teratogenicity studies to determine the potential for effects on the foetus, oral studies in which erythritol was administered over one or two generations to determine the potential for reproductive effects, and studies in bacterial and mammalian systems to determine mutagenic potential. The majority of the safety studies conducted were feeding studies in which erythritol was mixed into the diet at concentrations as high as 20%. The metabolic studies in animals have shown that erythritol is almost completely absorbed, not metabolized systemically and is excreted unchanged in the urine. The safety studies have demonstrated that erythritol is well tolerated and elicits no toxicological effects. The clinical program for erythritol involved a series of single-dose and repeat-dose, short-duration studies which have been used to investigate the human correlates to the physiological responses seen in the preclinical studies. The clinical studies showed erythritol to be well tolerated and not to cause any toxicologically relevant effects, even following high-dose exposure. Erythritol administered orally to humans was rapidly absorbed from the gastrointestinal tract and quantitatively excreted in the urine without undergoing metabolic change. At high oral doses, urinary excretion accounted for approximately 90% of the administered dose with minimal amounts appearing in the faeces. A comparison of the human and animal data indicated a high degree of similarity in the metabolism of erythritol and this finding supports the use of the animal species used to evaluate the safety of erythritol for human consumption. It can be concluded, based on the available studies that erythritol did not produce evidence of toxicity.

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.

An analysis of the effects of urethane on cardiovascular responsiveness to catecholamines in terms of its interference with Ca++ mobilization from both intra and extracellular pools

Urethane (1 X 10(-2) - 1 X 10(-1) M) reduced, in a concentration-dependent manner, both intra and extracellular Ca++ dependent noradrenaline-induced contractions of perfused rabbit ear artery as well as the tonic contractions produced by perfusion with high K+ solution. However, a quantitative analysis of the data indicated that for urethane concentrations similar to those found in plasma during anesthesia urethane antagonism is confined to noradrenaline-induced contractions which depend upon the mobilization of Ca++ from intracellular storage sites. In KCl-contracted arteries, urethane enhanced the relaxant effects of isoprenaline. - Urethane reduced the amplitude of contractions of spontaneously beating guinea-pig right atrium at concentrations which have only a limited effect on frequency. In addition, it decreased in a concentration-dependent manner the amplitude of isoprenaline-activated electrically driven, and K+ depolarized guinea-pig right ventricular strips. Urethane had no effect on the chrono and inotropic actions of isoprenaline on cardiac preparations. In in vivo experiments the chronotropic response to low doses of isoprenaline was significantly higher in urethane-treated as compared to unanesthetized rats. The higher dose of isoprenaline tested produced a significant fall in systolic blood pressure in urethane-anesthetized rats. A significant correlation exists between the chronotropic response to isoprenaline and resting heart rate values in urethane-anesthetized rats. These results indicate that urethane, at concentrations similar to those found in plasma during anesthesia selectively interferes with mobilization of Ca++ from intracellular storage sites. In addition, the interference of urethane anesthesia with the isoprenaline chronotropic effect 'in vivo' cannot be explained by a direct interference of urethane with beta-adrenoceptors at cardiac level.

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.

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.

Effect of hyperosmolarity on resting and developed tension in heart muscle

Simultaneous changes in weight, tension, and electrical activity were studied in the isolated perfused rabbit heart when the Ringer solution perfusion fluid was made hypertonic by the addition of sucrose, urea, glycerol, ethylene glycol, or formamide. The typical responses to each of the molecules was an initial drop in weight and tension followed by a return toward the base-line level. A 0.4 M concentration of sucrose, urea, or glycerol reduced the weight to 40 +/- 4.6, 48 +/- 3, and 52.2 +/- 3% of the initial value, respectively. The tension was simultaneously reduced to 23 +/- 3.5, 31 +/- 2, and 41 +/- 4% of its initial value. The tension drop produced by the solutes tested was linearly related to the amount of water lost by the heart. The falling phases of both tension and weight loss were closely correlated phases of both tension and weight loss were closely correlated in time a magnitude and were both related to the effectiveness of a particular molecule to move water out of the cells. In contrast, the subsequent rising phases of tension and weight were not as well correlated in time and magnitude. Whereas for most of the molecules used, the tension recovery was incomplete; urea, in contrast, caused an overshoot of the control tension level, thus pointing toward a unique inotropic effect of this compound. Resting tension rose for both urea and sucrose but not for the other compounds.

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.

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.