Urethane and contraction of vascular smooth muscle

Heart rate and blood pressure dependence of aortic distensibility in rats: comparison of measured and calculated pulse wave velocity

OBJECTIVES

When assessing arterial stiffness, heart rate (HR) and blood pressure (BP) are potential confounders. It appears that the HR/BP dependences of pulse wave velocity (PWV) and distensibility are different, even though both assess arterial stiffness. This study aims to compare aortic PWV as measured using pulse transit time (PWVTT) and as calculated from distensibility (PWVdist) at the same measurement site and propose a solution to the disparity in dependences of PWVTT and PWVdist.

METHODS

Adult anaesthetized rats (n = 24) were randomly paced at HRs 300-500 bpm, at 50 bpm steps. At each step, aortic PWVTT (two pressure-tip catheters) and PWVdist (pressure-tip catheter and ultrasound wall-tracking; abdominal aorta) were measured simultaneously while BP was varied pharmacologically.

RESULTS

HR dependence of PWVdist paradoxically decreased at higher levels of BP. In addition, BP dependence of PWVdist was much larger than that of PWVTT. These discrepancies are explained in that standard PWVdist uses an approximate derivative of pressure to diameter, which overestimates PWV with increasing pulse pressure (PP). In vivo, PP decreases as HR increases, potentially causing a PWVdist decrease with HR. Estimating the full pressure-diameter curve for each HR corrected for this effect by enabling calculation of the true derivative at diastolic BP. This correction yielded a PWVdist that shows HR and BP dependences similar to those of PWVTT. As expected, BP dependence of all PWV metrics was much larger than HR dependence.

CONCLUSION

Measured and calculated PWV have different dependences on HR and BP. These differences are, at least in part, because of approximations made in using systolic and diastolic values to calculate distensibility.

Heart rate dependence of aortic pulse wave velocity at different arterial pressures in rats

Arterial stiffness, as measured by aortic pulse wave velocity (PWV), is an independent marker of cardiovascular disease and events in both healthy and diseased populations. Although some cardiovascular risk factors, such as age and blood pressure, show a strong association with PWV, the association between heart rate (HR) and PWV is not firmly established. Furthermore, this association has not been investigated at different arterial blood pressures. To study effects of HR on aortic PWV at different mean arterial pressures (MAPs), adult (12 weeks; n=7), male, anesthetized Sprague-Dawley rats were randomly paced at HRs of between 300 and 450 bpm, at 50-bpm steps. At each pacing step, aortic PWV was measured across a physiological MAP range of 60 to 150 mmHg by infusing sodium nitroprusside and phenylephrine. When compared at the same MAP, increases in HR resulted in significant increases in PWV at all of the MAPs >80 mmHg (ANOVA, P<0.05), with the greatest significant change of 6.03±0.93% observed in the range 110 to 130 mmHg. The positive significant association between HR and PWV remained when PWV was adjusted for MAP (ANOVA, P<0.001). These results indicate that HR dependency of PWV is different at higher pressures than at lower pressures and that HR may be a confounding factor that should be taken into consideration when performing analysis based on PWV measurements.

In vivo Large-Scale Cortical Mapping Using Channelrhodopsin-2 Stimulation in Transgenic Mice Reveals Asymmetric and Reciprocal Relationships between Cortical Areas

We have mapped intracortical activity in vivo independent of sensory input using arbitrary point channelrhodopsin-2 (ChR2) stimulation and regional voltage sensitive dye imaging in B6.Cg-Tg (Thy1-COP4/EYFP)18Gfng/J transgenic mice. Photostimulation of subsets of deep layer pyramidal neurons within forelimb, barrel, or visual primary sensory cortex led to downstream cortical maps that were dependent on synaptic transmission and were similar to peripheral sensory stimulation. ChR2-evoked maps confirmed homotopic connections between hemispheres and intracortical sensory and motor cortex connections. This ability of optogentically activated subpopulations of neurons to drive appropriate downstream maps suggests that mechanisms exist to allow prototypical cortical maps to self-assemble from the stimulation of neuronal subsets. Using this principle of map self-assembly, we employed ChR2 point stimulation to map connections between cortical areas that are not selectively activated by peripheral sensory stimulation or behavior. Representing the functional cortical regions as network nodes, we identified asymmetrical connection weights in individual nodes and identified the parietal association area as a network hub. Furthermore, we found that the strength of reciprocal intracortical connections between primary and secondary sensory areas are unequal, with connections from primary to secondary sensory areas being stronger than the reciprocal.

A new procedure for the measurement of the outflow facility in conscious rabbits

A new procedure for measuring the outflow facility in conscious rabbits is described. The Langham pneumatic tonometer is applied horizontally against the eye; the intraocular pressure (IOP) is recorded before, during and immediately following 2 min of a pre-determined increased ocular pressure that is maintained at a fixed value by digital pressure applied through the eyelids. An increased volume of aqueous humor outflow resulting from the IOP increase is evaluated from the initial and final IOP values and the pressure volume relation for eyes of living rabbits. Close agreement in values of the outflow facilities in pairs of eyes of individual rabbits and excellent reproducibility of the procedure were found in repeated measurements made over a 24-hr period. The mean values of the IOP and the total outflow facility in 60 eyes of 30 rabbits were 20.5 +/- 0.2 mmHg and 0.17 +/- 0.01 microliter min-1 mmHg-1 respectively. Thirty minutes after an intravenous injection of acetazolamide, the IOP had decreased in both eyes of individual rabbits. This was associated with a decrease in the outflow facility and with a decrease of more than 50% in the rate of aqueous humor formation. One hour after the unilateral application of epinephrine the IOP had decreased in the treated eyes while the outflow facility remained unchanged.

Plasma protein extravasation induced by mammalian tachykinins in rat skin: influence of anaesthetic agents and an acetylcholine antagonist

The effect of mammalian tachykinins on plasma protein extravasation was assessed in the rat dorsal skin. Substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) increased vascular permeability in a dose-related manner with a threshold dose of about 0.07 pmol in sodium pentobarbitone-anaesthetized animals. Plasma protein extravasation induced by the tachykinins was 100-500 times less in magnitude in animals anaesthetized with urethane. Plasma protein extravasation induced by SP (66 pmol) was significantly reduced (63%; P less than 0.001) by atropine (a muscarinic inhibitor) while that induced by NKA or NKB was unaffected by the inhibitor suggesting that a cholinergic component might only be involved in the vascular permeability elicited by SP. The rank order of potency for the tachykinins on plasma protein extravasation was: NKB greater than SP greater than NKA (in absence of atropine) and NKB greater than NKA greater than SP (in presence of atropine), suggesting that this vascular response is mediated by a SP-E receptor type. The amplitudes of the plasma protein extravasation induced by NKB and its hydrophilic analogue [Arg degrees]NKB were similar, indicating that the lipophilic features of the native peptide cannot account for its potent biological activity. Plasma protein extravasation was enhanced by the SP analogue [D-Pro4,Lys6,D-Trp7,9,10,Phe11]SP (4-11), thus showing the limitation of such SP analogues (antagonists) for characterizing the tachykinin receptors involved in vascular permeability.

Peripheral and cerebrovascular actions of ethanol, acetaldehyde, and acetate: relationship to divalent cations

Inasmuch as ethanol is thought to exert its major effects in the autonomic and central nervous systems, it is important to determine whether acute versus chronic ingestion of this abused substance exerts any direct actions on peripheral and cerebral blood vessels. Since the chronic effects of ethanol on the cardiovascular system appear to be pivotal in the etiology of hypertension, coronary heart disease, and strokelike events, it is important to elucidate and understand the effects of chronic ethanol abuse and its mechanism(s) of action on the peripheral and cerebral blood vessels. Data are reviewed which suggest that the peripheral vasodilation and hypotension which result from acute ingestion (or administration) of ethanol may, in large part, be a consequence of its direct actions on vascular smooth muscle cells, both at the macro- and microcirculatory levels. At least two mechanisms appear to contribute to this vasodilator effect: inhibition of the normal rhythm or vasomotion (spontaneous mechanical activity) of vascular smooth muscle, and depression of the contractile responses to endogenous neurohumoral substances that play a role in maintaining vascular tone and regulation of blood flow. The data acquired so far suggest that the dilator actions are related causally to interference with movement and/or translocation of Ca2+ across the vascular membranes. In addition, these actions appear to resemble the peripheral vascular effects of general anesthetics. Evidence is also reviewed which indicate that ethanol, in contrast to acting as a vasodilator in the splanchnic vasculature, is often a potent and concentration-dependent constrictor of arterioles and venules in the skeletal muscle vasculature. Direct in situ observations on the rat brain, using high resolution, quantitative TV image-intensification microscopy, indicates that administration of ethanol, irrespective of the route of administration (e.g., perivascularly, intraarterially, or systemically), produces graded concentration-dependent spasms of arterioles and venules. Concentrations of ethanol approximately equal to greater than 250 mg/dl produce intense spasms resulting in rupture of these vessels. Recent in situ studies in conscious dogs, using radiolabeled microspheres, also indicate that ethanol can produce deficits in regional brain blood flow. Studies with isolated canine middle cerebral and basilar arteries clearly demonstrate that low concentrations of ethanol (e.g., less than 10 mM) can produce concentration-dependent spasms by a direct vascular action.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of diethyl ether, halothane, ketamine and urethane on sympathetic activity in the rat

The present paper describes the effects of different general anaesthetics on plasma catecholamine (CA) concentrations taken as biochemical index of peripheral sympathetic activity. In chronically catheterized rats, diethyl ether, ketamine and urethane increased plasma adrenaline (A) and noradrenaline (NA) concentrations, indicating that these drugs stimulate both neurosympathetic and adrenomedullary functions. These effects appear to be centrally mediated, since ganglionic blockade or spinal transection completely counteracted the diethyl ether- and ketamine-induced increases in plasma CA levels. Halothane induced a transient decrease in circulating A and an increase in NA. These results support the concept that general anaesthetics may have different effects on sympathetic function. Arterial blood pressure and heart rate were also measured to look for possible correlations with peripheral sympathetic activity. The enhanced release of peripheral CAs seemed to be the determining factor for increasing blood pressure and heart rate with ketamine only. In the other instances the activation of the peripheral sympathetic system appeared to maintain homeostasis by counterbalancing the various depressive effects of anaesthetics on the cardiovascular system.

Arteriolar vasomotion and arterial pressure reduction in rabbit tenuissimus muscle

Spontaneous arteriolar vasomotion and its relation to arterial pressure reduction was studied in the rabbit tenuissimus muscle using intravital microscopy. Vasomotion was observed in all transverse arterioles and their first-order side branches. Vasomotion frequency ranged from 5 to 32 cycles per minute (median: 25 cpm). The relative vasomotion amplitude in transverse arterioles varied from 0.06 to 0.44 (median: 0.22). Vasomotion was generally of the on-off type in first-order side branches. A gradual reduction in arterial pressure as achieved by aortic occlusion resulted in an abrupt disappearance of vasomotion. Vasomotion disappeared between 19 and 59 mm Hg (median: 30.4 mm Hg). No differences were found between the pressures at which vasomotion ceased in transverse arterioles and first-order side branches. With a further reduction of arterial pressure transverse arterioles attained a maximal diameter of 98-265% (median: 119%) of the maximal diameter during vasomotion. After release of occlusion vasomotion reappeared much earlier in the first-order side branches than in their feeding transverse arterioles. It is concluded that although vasomotion influences capillary perfusion, it does not play a major role in the adaptation of vascular resistance following a reduction in arterial pressure.

Ethanol increases single unit activity in the inferior olivary nucleus

Single unit recording of rat inferior olivary nucleus neurons reveals significantly elevated discharge after acute intraperitoneal injection of 2 g/kg ethanol. This effect is consistent across 3 different methods of anesthesia and immobilization: local Xylocaine plus intraperitoneal D-tubocurare, intraperitoneal chloral hydrate and halothane vapor. In contrast, under urethane anesthesia acute ethanol produces significant depression of olivary discharge. Since this effect is opposite to that found under the other anesthetic conditions (including topical Xylocaine only), urethane anesthesia may compromise generalizations of electrophysiologic studies of ethanol. Neurons of the inferior olivary nucleus excite cerebellar Purkinje cells through a powerful afferent circuit; our data therefore suggest that ethanol-induced increases in cerebellar Purkinje cell complex (climbing fiber burst) spikes, obtained in our previous studies, are secondary to olivary activation.

Suitability of urethane anesthesia for physiopharmacological investigations in various systems. Part 2: Cardiovascular system

Urethane produces a level of surgical anesthesia characterized by preservation of a number of cardiovascular reflexes. When the proper route of administration is used, and the use of unnecessarily high doses is avoided, urethane anesthesia appears to be suitable for a number of investigations at cardiovascular level. However in certain types of studies involving pharmacological stimulation of peripheral adrenoceptors urethane affects markedly the magnitude of the response under study.

The effect of urethane and thiopental sodium on platelet aggregation in vitro and in vivo

The potential in vitro (heparinized or citrated PRP) and in vivo effects of urethane and thiopental sodium on arachidonic acid, collagen, or ADP-induced rat platelet aggregation has been investigated. Both anesthetics antagonized platelet aggregation in vitro at concentrations higher than those found in plasma during anesthesia. Neither anesthetic altered the piastrinopenia induced by intravenous administration of these aggregating agents. These findings suggest that both anesthetics are suitable for in vivo platelet aggregation studies.

The effect of nifedipine and verapamil on KC1-induced rhythmic contractions of guinea pig ureter in vitro

Addition of KC1 (40 mM) produced rhythmic contractions of guinea-pig ureters in vitro which were unaffected by phentolamine, atropine or tetrodotoxin. KC1 failed to elicit rhythmic contractions of ureters incubated in a Krebs solution with no added Ca++; in these conditions the addition of CaC12 in concentrations of 1.5 mM, or higher, produced rhythmic contractions whose frequency, but not amplitude, was proportional to CaC12 concentration in the bathing medium. EDTA reduced the frequency of KC1-induced rhythmic contractions without affecting their amplitude. Nifedipine and verapamil reduced both the frequency and the amplitude of KC1-induced rhythmic contraction; verapamil was more effective than nifedipine in reducing their amplitude. Urethane reduced the amplitude without significantly affecting the frequency of KC1-induced rhythmic contractions. An increase in the extracellular Ca++ concentration reverted the suppressive effect of all drugs under study. These results suggest that an influx of Ca++ from the extracellular space is responsible for the initiation of KC1-induced rhythmic contractions and is involved in the mechanism(s) which regulates their frequency, but that a separate mechanism regulates their amplitude.

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.

Influence of primary prostaglandins on isolated canine renal arteries and veins

The sensitivity and contractility of isolated canine renal arteries (RA) and renal veins (RV) to primary prostaglandin compounds (PG) was studied. Studies were also undertaken to determine whether specific receptors for PGs exist in RA and RV. RA and RV exhibited potent concentration-dependent contractile responses to all the primary PGs studied, including PGA1, PGA2, PGB2, PGD2, PGE1, PGE2, PGF1 alpha and PGF2 alpha. The contractile sensitivity (based on EC50's) of canine RA to PGs was: PGB2 greater than PGB1 approximately equal to PGE2 greater than PGF2 alpha approximately equal to PGA2 approximately equal to PGD2 approximately equal to PGA1 greater than PGF1 alpha greater than PGE1, whereas for RV it was: PGB2 greater than PGB1 approximately equal to PGD2 approximately equal to PGF2 alpha greater than PGA2 approximately equal to PGA1 greater than PGE2 much much greater than PGF1 alpha approximately equal to PGE1. In terms of the ability to generate a maximum contractile response on RA, PGB2 was the most potent and PGD2 the least potent, whereas for RV PGF2 alpha and PGB2 was the most potent and PGF1 alpha and PGE1 the least potent. Canine RA failed to elicit any consistent relaxant responses to PGE1, PGE2, PGA1, and PGA2. Renal veins, however, in which tone was induced by either PGs or serotonin responded with concentration-related relaxations to PGE1; other primary PGs did not induce relaxations on isolated RV. Use of specific pharmacologic antagonists (for catecholamines, serotonin, acetylcholine and histamine) failed to interfere with any of the PG responses. The data indicate that RA and RV: a) can exhibit differential responses to primary PGs; b) exhibit structure-activity relationships for the contractile action of PGs; and c) appear to have specific receptors for primary PGs which primarily subserve contraction.