Vascular flow resistance in rabbit hearts: "Apparent viscosity" of RBC suspensions

Isovolumically beating, isolated hearts from male New Zealand rabbits were perfused retrograde via the aorta at a paced rate of 180 to 200 beats per minute. Perfusions were effected at arterial pressures (AP) ranging from 10 to 80 mm Hg. Perfusions (Krebs-Henseleit buffer + 1% BSA + insulin, 100 μl/liter; same + 40% sheep RBC; same + 40% bovine RBC) were dosed with 15 mM papaverine to induce maximum vasodilation. In each case normalized flow rate (per unit heart mass) varied linearly with AP but all linear regressions extrapolated to a positive, zero-flow value of AP. Normalized flow resistance, defined as the slope of the line fitted to the pressure-drop vs normalized flow (per unit of dry heart weight) data, was not affected by the change from sheep to bovine RBC in the maximally dilated hearts. Data from RBC-free perfusions were inserted in Poiseuille's law to compute an effective geometric factor for the organ vasculature. This is turn was used to calculate apparent viscosities of the RBC suspensions. These were indistinguishable for the different RBC suspensions and fell between 0.5 and 0.6 of the respective high shear-rate values measured in a cone-plate viscometer, thus agreeing with the findings of S.R.F. Whittaker and F. R. Winton (1933, J. Physiol. (London), 78, 339-369) for the dog hindlimb. Relative apparent viscosities (suspension viscosity ÷ suspending medium viscosity) were 1.89 (sheep RBC) and 1.85 (bovine RBC) and are in good agreement with the range deduced for the mesenteric microcirculation of the cat from the microscopic measurements of Lipowsky et al. (1980), signifying that the low apparent in vivo viscosity determined for an intact organ follows from the microhematocrit deficiency.