Capillary and cell wall permeability to potassium in isolated dog hearts

Abstract

From venous tracer-dilution curves recorded after 36 pulse injections of 42KCl and 131I-labeled albumin into the coronary artery inflow of 15 isolated canine heart preparations, we calculated maximal fractional extractions (Emax) and capillary permeability-surface area products (PScap) for 42K+ over a range of plasma flows (FP) from 0.3 to 1.7 ml min-1 g-u. At low FP (less than 1.0), Emax was 0.60 +/- 0.0l (mean +/- SD) and PScap was 0.72 +/- 0.20 ml min-1 g-1; at high FP (greater than 1.0), Emax decreased to 0.49 +/- 0.05 and PScap increased to 1.06 +/- 0.18. Continuous recording (gamma detector) of residual myocardial 42K+ in seven hearts showed that the mean fractional escape rate of tracer between 30 and 60 min after injection was 0.011-0.023 min-1; higher rates were observed at high FP, when the residue of 42K+ decreased to less than 10% of the injected dose by 60 min. Using PScap measured at high FP and considering the virtual intracellular volume of distribution for K+ to be 20 ml/g, we calculated the permeability-surface area product for sarcolemma (PScw) as 0.54-0.73 ml min-1 g-1, or about 50% of PScap. Considering sarcolemmal surface area (Scw) as 4,200 cm2/g and capillary surface area (Scap) as 500 cm2/g, cell permeability is low, with Pcw:Pcap being less than 0.08.

FIG. 1

Experiment 29059. Venous concentration vs. time curves for ¹³¹I-labeled albumin [h_R(t)] and ⁴²K⁺ [h_K(t)], each normalized to fraction of injected dose. At high flow (left), a greater fraction of injected dose is recovered in a shorter period and curves are less temporally dispersed than at low flow (right). Fractional extractions [E(t), broken lines] increase to maximum (E_max) at or slightly before time of peak of h_R(t). E_max is higher at low flow and persists for approximately 4 s before decrease in E(t) is seen.

FIG. 2

Effect of plasma flow on maximal fractional extraction (E_max, top) and on capillary permeability-surface area product (PS_cap, bottom) for ⁴²K⁺ in isolated blood-perfused canine hearts. Data from Table 2 are plotted as open circles. E_max decreased as F_P increased. PS_cap (from equation 2a) increased with increases in F_P, but a definite plateau was not seen over range of F_P examined.

FIG. 3

Washout of ⁴²K⁺ from isolated heart after single injection of ⁴²KCl into aortic root, monitored by a detector directly over left ventricle. Data are plotted in counts per minute on a linear scale (top) and a semilog scale (middle). When washout becomes monoexponential, emergence function (bottom) reaches a constant minimum.

FIG. 4

Residue functions (top) representing fraction of injected ⁴²K⁺ remaining in heart at any time after intra-arterial injection, for each of 7 ⁴²K⁺ efflux studies summarized in Table 2. At any given time, residue of ⁴²K⁺ was less at higher plasma flows (F_P). During early phase of washout, mean fractional escape rate of tracer, emergence function (bottom), between 30 and 60 min after injection ranged from 0.011 to 0.023 with higher values observed at higher F_P. After 160 min, emergence functions decreased to less than 0.010, and no differences were seen between studies done at high and low F_P.