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Biomedical subjects

R H Cox

Publications and source records attributed to R H Cox.

At least 163 records · Page 9Linked to original sources
PubMed1976

Comparison of arterial hemodynamics in the mongrel dog and the racing greyhound.

This study was undertaken to define certain differences in cardiovascular function between mongrel (M) and greyhound (GH) dogs. In unanesthetized, trained, chronically instrumented animals mean arterial pressure was significantly higher in the GH (118 vs. 98 mmHg). This was associated with a significantly higher cardiac index in the GH (4.3 vs 3.1 liters/min per m2) and a lower calculated peripheral resistance. Central venous renin activity was significantly lower in the GH when unanesthetized (1.51 vs. 2.88 ng/ml per h). Values of vascular impedance at several selected arterial sites were lower in the GH compared to the M, suggesting differences in arterial wall mechanical properties. The hydraulic power delivered to the aorta by the left ventricle in the GH was twice that of the M (1,166 vs. 564 mol wt). Oscillatory power represented a greater fraction of total aortic power in the GH (15.7 vs. 10.8%). Differences were also observed in the hemodynamic response to acute pentobarbital anesthesia. Thus the young adult GH is hemodynamically different from its mongrel counterpart. These differences bear some resemblance to hemodynamic changes seen in various types of experimental hypertension in animals and to those observed in the early phase of essential hypertension in man.

Animals↗
PubMed1976

Mechanics of canine iliac artery smooth muscle in vitro.

Transmural pressure and external diameter were continuously recorded from intact segments of canine iliac artery and used to determine wall responses to maximal smooth muscle (SM) activation by norepinephrine (NE). Three different approaches were used:a) step inflations from low pressure following NE, B) slow, continuous inflation from low pressure following NE, and c) direct isometric and isobaric responses to NE. Passive pressure-diameter data were determined. The results show that active wall stress and diameter responses to NE determined using direct isometric and isobaric responses and from slow inflation responses (less than 1 mmHg/s) were not significantly different. Pressure-diameter and stress-strain curves determined using continuous (0.2 mmHg/s) and step pressure inflations after NE were not signficantly different. Pressure-diameter curves obtained at different inflation rates were not those expected from a simple viscoelastic material. The results suggest that slow inflation of iliac arteries with activated SM is a reasonable method for assessing contractile properties of SM. Furthermore, they also suggest that activation of iliac artery SM at high contractile element length (L greater than Lo) does not produce attenuated constriction responses.

Animals↗
PubMed1976

Mechanics and electrolyte composition of arterial smooth muscle in developing dogs.

The effects of age on arterial smooth muscles (SM) mechanics and electrolyte composition were studied during growth and development in six litters of puppies and their mothers from isolated segments of carotid, renal, mesenteric, and iliac arteries. An increase in the maximum wall stress following both norepinephrine (NE) and potassium (K) was found with age at all sites. The ratio of the stress response for K/NE was found to decrease with age. The maximum stress response for both NE and K shifted to lower values of wall strain with age in the same manner that passive stress-strain curves were shifted. The maximum diameter re3 (3 SM activator to SM activator were not significantly altered with age, but they were better maintained at higher transmural pressure in older animals. Chemical analysis indicated "cell" water, Mg, and K contents decreased with age, as did extracellular water, Na, and Cl. Na anhe suggest that changes in SM excitation-contraction coupling occur with age. Passive wall elements appear to interact with the contractile elements in determining wall responses to SM activation.

Aging↗
PubMed1976

Effects of norepinephrine on mechanics of arteries in vitro.

The effects of smooth muscle activation on the pressure dependence of arterial wall characteristic impedance were studied with isolated segments of canine iliac and carotid arteries. Measurements of external diameter and transmural pressure were made before and after activation of the arterial smooth muscle (SM) by norepinephrine (NE) in concentrations of 0.5 and 5 mug/ml and used to compute values of characteristic impedance (Z0). In the absence of SM tone, values of Z0 for both arterial sites increased monotonically with transmural pressure. For the larger [NE], values of ZO exhibited a minimum at pressures of the order of 125 mmHg and increased for both larger and smaller values of pressure, For the smaller [NE], values of Z0 showed a similar pressure dependence but with a broader minimum. It is concluded that the previously demonstrated constancy of vascular impedance with changes in arterial pressure is at least the result of the presence of a tonic level of SM activation in conduit arteries.

Animals↗
PubMed1976

Hemodynamic responses to stellate ganglion stimulation in mongrels and greyhounds.

A comparison of the effects of left stellate ganglion stimulation (SGS) on central and aortic hemodynamics has been made in chloralose-anesthetized mongrel (M), and greyhound (GH) dogs. Measurements of aortic pressure and flow, and left ventricular pressure were made during stimulation of the decentralized left SG at different frequencies from 0 to 20 Hz. The increases in aortic pressure and flow with SGS were larger in the GH, especially for low frequencies of stimulation. Stroke volume was increased with SGS in the GH at all stimulation rates, whereas in the M it was unchanged. A greater decrease in left ventricular end-diastolic pressure with SGS was found in the GH. These results suggest that differences exist in both the intrinsic and extrinsic control of cardiac output in the greyhound dog compared to the mongrel. These differences may be in part responsible for the elevated arterial blood pressure in the greyhound compared to the mongrel.

Adrenergic Fibers↗
PubMed1976

Effect of thoracotomy upon the carotid sinus control of regional hemodynamics in the dog.

The effects of bilateral thoracotomy upon the operating characteristics of the carotid sinus reflex were examined in 11 vagotomized dogs. Pressure-flow relationships in the aorta, celiac, mesenteric, renal, and femoral arteries were studied as a function of pressure in the isolated carotid sinuses. These relationships were analyzed to give the carotid sinus reflex operating point values and sensitivities of pressure, flow, impedance, and power for each regional vascular bed. Following thoracotomy there was a significant fall in the closed-loop operating point pressure (CLOP) ( p less than 0.005). The operating point values and sensitivities for each regional flow and resistance together with the corresponding mean and oscillatory kinetic and pressure power were not significantly altered by thoracotomy. Renal characteristic impedance at the operating point significantly increased following thoracotomy (p less than 0.05), as did the aortic characteristic impedance for carotid sinus pressures (CSP) above the closed-loop operating point. The operating sensitivity of renal characteristic impedance was decreased (p less than 0.05). Possible mechanisms underlying these changes are discussed.

Animals↗
PubMed1975

Altered arterial connective tissue in racing greyhound dogs.

Vascular collagen and elastin contents and the ratio of collagen/elastin (C/E) were studied in racing greyhound dogs, a breed which exhibits increased cardiac output. As compared to mongrel dogs, vascular C/E was lower, suggesting a greater distensibility of vessels as an adaptive response to hemodynamic stress.

Animals↗
PubMed1975

Pressure-flow relations in the vessels of the canine aortic arch.

Simultaneous measurements of pressure and flow have been made in the vessels of the aortic arch in chloralose-anesthetized dogs under three conditions; control, cardiac sympathetic (SS), and vagal nerve stimulation (VS). The specific sites studied include ascending aorta (AA), descending thoracic aorta (DTA), and brachiocephalic (BCA), left subclavian (LSC), and common carotid arteries (CA). Recorded data were analyzed by Fourier series methods and were used to calculate vascular impedance spectra, pressure and flow propagation characteristics, and hydraulic power. The distribution of cardiac output was found to be 70% to DTA, 22.2% to BCA, and 6.5% to LSC. The fraction of cardiac output to the BCA, LSC, and CA was increased by both SS and VS. The hydraulic power was distributed to the arch vessels nearly the same as the cardiac output. Hydraulic power dissipated in the arch vessels averaged 6% of the total AA hydraulic power and was increased by SS and decreased by VS. The results suggest that viscous losses in the arterial walls represent a considerable portion of the power dissipation. A similarity found in the vascular impedance spectra at the various sites for all three conditions was also observed for the transmission characteristics of pressure and flow harmonics between the sites. The results suggest that major reflections do not occur in the proximal aorta.

Animals↗
PubMed1975

Arterial wall mechanics and composition and the effects of smooth muscle activation.

A comparison of passive mechanics and the effects of smooth muscle (SM) activation has been made in canine carotid and iliac arteries. Measurements of external diameter and axial force were made on isolated cylindrical segments in response to internal pressure variations. Data were obtained during maximal norepinephrine (NE) and potassium (K) activation, and following metabolic inhibition of SM. These data were used to determine wall tangential stress-strain relations. The maximum increase in wall stress after K and after NE was greater for the iliacs and occurred at smaller values of wall strain. Passive stress-strain curves for the iliacs were likewise shifted to smaller strains, suggesting an important role of passive wall elements in setting the length of contractile elements. Diameter responses to K at low values of pressure were the same for iliacs and carotids, but were better maintained at higher pressure levels for the iliacs. Similar findings were found for iliac responses to NE and K. The results suggest that at low values of wall stress, active diameter responses are not strongly dependent on the maximum isometric stress development. However, higher pressure-diameter responses are also determined by the force-generating capacity of SM.

Animals↗
PubMed1975

Pressure dependence of the mechanical properties of arteries in vivo.

Simultaneously measurements of intra-arterial pressure and external diameter were recorded from the thoracic and abdominal aortas, the carotid, brachiocephalic, left subclavian, and femoral arteries of anesthetized dogs. Data were recorded under control conditions and during efferent vagal stimulation, and were used to compute values of wall geometry, mechanical properties, phase velocity, and characteristic impedance. Values of these parameters computed for both control and vagal stimulation were used to represent their dependence on mean arterial pressure. Both the dynamic elastic modulus and the phase velocity increased with mean pressure at each arterial site. Values of computed characteristic impedance were constant and independent of mean arterial pressure between 80--150 mmHg. The tropical application of norepinephrine (100 mug/ml) to the femoral artery produced no change in mean pressure, while external diameter and dynamic modulus were decreased, and characteristic impedance was increased. These results provide direct support for the conclusion of previous studies on vascular impedance spectra which suggested a constancy of characteristic impedance with changes in mean arterial pressure.

Acetylcholine↗
PubMed1975

Effects of carotid hypotension on aortic hemodynamics in the unanesthetized dog.

The effects of occlusion of the brachiocephalic artery on aortic hemodynamics were assessed in 12 chronically instrumented dogs in the unanesthetized state. Continuous measurements of ascending aorta pressure and flow were made. In the steady state following occlusion, heart rate increased by 36% and mean arterial pressure by 45%, while cardiac output was unchanged from preocclusion levels. Hydraulic power delivery to the systemic circulation by the left ventricle was increased during occlusion, while the fraction of total power associated with pulsations decreased. Values of peripheral resistance and ascending aorta input impedance were both increased during occlusion. Graded occlusions of the brachiocephalic artery produced graded, monotonic increases in the entire aortic impedance spectrum between 2 and 20 Hz with more sensitive responses occurring with the smaller, submaximal responses. Considered with results of previous studies, these results suggest that activation of smooth muscle in large conduit arteries is also associated with the pressor response which accompanies carotid hypotension and that such activation has a hemodynamically significant effect.

Animals↗
PubMed1975

Baroreceptor reflex control of arterial hemodynamics in the dog.

The regional differentiation of carotid sinus control of arterial pressure-flow relationships was studied in chloralose-anesthetized dogs. Simulatneous pressure-flow measurements were made in the ascending aorta, the celiac artery, the cranial mesenteric artery, the renal artery, and the femoral artery. The carotid sinuses were bilaterally isolated and perfused with pulsatile pressure. The open-loop reflex gain was not symmetrical about and was maximum at pressures below the closed loop operating point pressure. Changes in both peripheral resistance and cardiac output contributed significantly to the open-loop gain with the former predominating. Aortic impedance for frequencies above 3 Hz was at a minimum at the closed-loop operating point and increased for both higher and lower values of carotid sinus pressure. For the frequency range from 3 to 9 Hz, regional impedance in all of the beds varied inversely with carotid sinus pressure. The sensitivity of the various beds to changes in carotid sinus pressure around the operating point increased in the order celiac less than mesenteric less than renal less than femoral. Following vagotomy, operating point values of regional resistance and sensitivity were significantly increased. This fact suggests that the aortic arch receptors exert a significant influence on regional vascular impedance at operating point pressures. The fraction of cardiac output in the celiac, mesenteric, and renal beds was nearly independent of carotid sinus pressure before and after vagotomy, but that in the femoral bed increased with carotid sinus pressure. These results demonstrate the nonuniform nature of carotid sinus and aortic arch baroreceptor control of regional blood flow.

Animals↗
PubMed1975

Differential effects of alphaMT on anorectic and stimulatory action of amphetamines.

Pretreatment of rats with a dosage regimen of alphaMT that has no effect on the anorectic action of a single dose of 2 mg/kg of d-amphetamine or methamphetamine causes a marked reduction in the rate of continuous avoidance responding evoked by that same dose. Similar pretreatment followed by a dose of 8.0 mg/kg of benzphetamine was without effect on the drug's action on both systems.

Amphetamines↗