Reference sample method for cardiac output and regional blood flow determinations in the rat

The reference sample method was used for simultaneous determinations of cardiac output and regional blood flows in conscious rats. Microspheres (15 +/- 5 mum in diam) labeled with strontium-85 were injected into the left ventricle and known volumes of reference sample were withdrawn from peripheral arteries. The calculated cardiac output measurements agreed with the previously reported values in rats. The percent distribution of the cardiac output to the brain, intestinal bed, and lungs were different from the reported values obtained in the rats using larger spheres. These differences may be related to the use of smaller spheres and to differences in the preparation. The absolute regional flows to various organs expressed in terms of milliliters per minute per gram tissue weight were also determined. The results indicate that the reference sample method can be applied to smaller mammals for determinations of regional flows and cardiac output.

Pulmonary arterial wedge and left atrial pressures and the site of hypoxic pulmonary vasoconstriction

The pulmonary vascular response to breathing 5% oxygen in nitrogen was studied in anesthetized, artificially ventilated dogs. The pulmonary arterial (Pa), pulmonary arterial wedge (PW), left atrial (LAP) pressures and pulmonary blood flow (Q) were monitored. The pulmonary arterial catheter was wedged at the mid-lung level. At 10 min of hypoxia, Pa-PW pressure gradient increased while the PW-LAP gradient did not change significantly. The PW and LAP were significantly correlated during room air breathing (r=0.832) and during hypoxia (r=0.980). The calculated resistance from the pulmonary artery to the wedged catheter (Pa-Pa/Q) increased and the calculated resistance from the wedged catheter (Pa-Pw/Q) did not change significantly. These findings indicate that the mean LAP and PW pressures are not significantly different during severe hypoxia, and that the increase in pulmonary vascular resistance during hypoxia is due to constriction of the large precapillary vessels.

Adrenergic mechanisms and the pulmonary vascular response to respiratory acidosis

The role of sympathetic mechanisms in mediating the pulmonary vasoconstrictor response to respiratory acidosis was studied in intact dogs. Arterial oxygen tension and ventilation were maintained at resting levels and the response was studied during a constant level of alpha- and beta-adrenergic blockade. There were significant increases in the pulmonary vascular resistance (PVR) and pulmonary perfusion pressure and no change in pulmonary blood flow (Q) when the dogs breathed 5% CO2 for 10 min. The alpha-adrenergic blocking agent, phenoxybenzamine, did not significantly alter the pulmonary vascular response, while the beta-adrenergic blocking agent, propranolol, enhanced the response. Phenoxybenzamine significantly reduced the resting pulmonary perfusion pressure from control values, while propranolol did not alter it. Both propranolol and phenoxybenzamine produced comparable decreases in the resting Q from control values. The resting PVR increased to a greater extent with propranolol than with phenoxybenzamine. These results indicate that adrenergic mechanisms do not play a role in mediating rise in PVR induced by respiratory acidosis. The finding that the pulmonary vasoconstrictor response to respiratory acidosis is enhanced during beta-adrenergic blockade suggests that vasoconstrictor alpha-receptors may be unmasked during beta-adrenergic blockade. Finally, the studies suggest that both alpha- and beta-receptors contribute to maintaining the resting PVR.

Role of adrenergic mechanisms in the development of cardiac hypertrophy

This experiment was designed to study the role of cardiac beta-adrenergic mechanisms in the development of hypertrophy in rats. The suprarenal abdominal aorta was banded, resulting in an increase in cardiac wt-body wt ratio. A group of rats received a sham operation. Half of the banded rats were treated with practolol, 2.0 mg/kg intraperitoneally every 12 hr for the 6 days after banding. The effectiveness of cardiac beta-adrenergic blockade was confirmed by absence of an increase in heart rate following intravenous isoproterenol at various times between practolol injections. Practolol did not affect the gradient in the banded groups. Six animals in each banded group were sacrificed daily for 6 days. The right and left ventricles were dissected separately and weighed. RV-body weight ratios increased similarly in both banded groups. LV-body weight ratio (g/kg) was 2.17 +/- 0.043 in sham rats, and it attained maximal levels of 3.03 +/- 0.10 within 6 days in banded untreated rats and 2.96 +/- 0.14 in banded rats receiving practolol. Therefore, beta-adrenergic mechanisms were not involved in the development of hypertrophy due to increased afterload. Also, these findings are not consistent with the Meerson hypothesis, since hypertrophy occurred despite the reduction in myocardial O2 consumption due to practolol.

The independent effects of changes in H + and CO 2 concentrations on pulmonary hemodynamics of intact dogs

The hydrogen ion concentration ([H+]) and the carbon dioxide tension [Formula: see text] of the arterial blood were varied independently of the other over a range of values while the respiration was controlled at resting levels. The independent effects of the two variables were examined in intact dogs anesthetized with 30 mg/kg sodium pentobarbital. Increases in [H+] and [Formula: see text] were significantly correlated (r = 0.859 and r = 0.646) with increases in the pulmonary vascular resistance (P.V.R.). The rise in P.V.R. in both cases was associated with rise in pulmonary perfusion pressure and no significant change in pulmonary blood flow. Decrease in [Formula: see text] had no effect on the measured parameters. We have demonstrated the independent pulmonary vasoconstrictor effects of increase in the [H+] and [Formula: see text]. The magnitude of the pulmonary vasoconstrictor response is dependent upon the degree of rise in [H+] and [Formula: see text]. [Formula: see text] does not play a role in maintaining resting pulmonary vasomotor tone.