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Repeated blood sampling (0.2 ml) in the conscious rat during the course of 10 cardiac index measurements using the Fick procedure did not alter the cardiac index as measured initially (285 ml x min-1 x kg-1). However, oxygen consumption and hematocrit were reduced 7-19% and 4-14%, respectively. Replacement of blood removed during sampling with donor blood prevented these responses, but also led to reduced cardiac index and arterial oxygen content, 22-28% and 10-21%, respectively. In additional studies in anesthetized rats, hemorrhage (25 ml/kg) increased plasma K+ by 29% and reduced plasma Na+ by 3%, suggesting that compensatory fluid replacement originated in cells as well as interstitium. This fluid replacement after blood loss helps sustain normal systemic hemodynamics, but blood loss can produce metabolic alterations that should be taken into account in any biochemical study. Although metabolic alterations can be prevented by replacing lost blood with donor blood, cardiopulmonary function may be adversely affected.

Body fluid volumes were examined in young spontaneously hypertensive rats (SHR) and in two strains of age-matched normotensive controls to ascertain whether expanded plasma volume (PV) or extracellular fluid volume (ECFV) was associated with onset of spontaneous hypertension. Mean arterial pressure (MAP) was measured in conscious male SHR, Wistar-Kyoto normotensive (WKY), and American Wistar normotensive (NR) rats 3 h after arterial cannulation. At age 10-14 days no differences in MAP or PV (determine with 125I-albumin) were found between SHR and WKY. At age 18-43 days, SHR had elevated MAP and a small but significant elevation in total body water volume (TBWV; wet weight minus dry weight) compared to WKY and NR; no differences among the three groups were found in PV or ECFV (estimated with inulin). These results provide no evidence that expanded PV or ECFV plays a role in the pathogenic mechanisms of spontaneous hypertension, but do not exclude the possibility of altered vascular compliance. The slightly elevated TBWV in SHR may be related to reduced body fat in SHR.

Conscious spontaneously hypertensive rats (SHR), 5--7 wk old, were studied hemodynamically by the direct Fick procedure to determine whether high total peripheral resistance (TPR) coexisted with increased oxygen consumption (QO2) at an early stage of hypertension development. Since under resting conditions cardiac output in SHR was not significantly different from normotensive controls, the elevated arterial pressure and QO2 were associated with increased TPR. Arterial hypoxemia was induced to reduce oxygen availability and to assess whether increased TPR in SHR could be reversed by this procedure. During hypoxemia, normotensive controls (WKY) responded with increased cardiac output and decreased arterial pressure and TPR. In contrast, arterial pressure and cardiac output fell in SHR; and the increased TPR persisted. QO2 fell in hypoxemic SHR demonstrating that the relationship between total body oxygen consumption and cardiac output was abnormal in young SHR, and that increased TPR in SHR was not dependent on resting levels of QO2 or oxygen availability. Although QO2 was elevated in SHR compared to age-matched WKY, this condition was not essential for maintained elevated vascular resistance.

Regional blood flow measurements made by the radioactive microsphere technique were studied in conscious rats. A femoral arterial reference sample blood flow was measured directly, and at the same time indirectly by the combined use of direct Fick cardiac output and microsphere techniques. A significant correlation (r = .81, P less than .01) was obtained between direct and indirect blood flow values when 200--400 microspheres were trapped in the reference sample. When 100--200 microspheres were trapped, regional blood flow was 32% below true flow (P less than .01); and cardiac output, calculated by the reference sample method, was 57% greater than Fick cardiac output (P less than .01). When three consecutive Fick determinations and microsphere injections (20,000 per injection, 15 micrometer diam) were made in conscious rats, significant correlations were obtained among the first, second, and third regional blood flow measurements (r = .95, P less than .01). The results have demonstrated that cardiac output and reference blood flow can be measured with accuracy and precision in the conscious rat by the radioactive microsphere procedure.

The effect of increasing quantities of carbonized microspheres (15 +/- 5 micrometer diam) on systemic hemodynamics was evaluated in 21 Sprague-Dawley, 6 normotensive Wistar-Kyoto (WKY), and 8 spontaneously hypertensive (SHR) male rats. Total cumulative injections of more than 100,000 microspheres produced significant systemic hemodynamic alterations manifested by decreased oxygen consumption, cardiac output, and mean arterial pressure; and by increased heart rate, total peripheral resistance, and arterial-venous blood oxygen content difference. There were no hemodynamic alterations characteristic of any rat species, and reproducibility of three separate injections of 20,000 microspheres in each rat was excellent.