Increased flow rate and papaverine increase K+ exchange in perfused rat hind-limb skeletal muscle

This study tested the hypothesis that increases in perfusate flow rate result in increased rates of unidirectional and net K+ transport in rat hind-limb skeletal muscle at rest. Ten neurally and vascularly isolated hind limbs, with arterial and venous catheters placed proximal to the popliteal region, were perfused for 10-min periods at flow rates (presented in a random order) of 0.27, 0.42, 0.63, 0.84, or 1.05 mL·min-1·g-1. Potassium extraction and unidirectional K+ influx were determined using 42K, and arterial perfusion pressure was measured continuously. Increases in flow rate resulted in decreases in K+ extraction and increases in unidirectional K+ influx, unidirectional K+ efflux, and net K+ efflux. The increases in K+ flux were associated with increases in oxygen uptake, glucose uptake, and lactate release. In separate experiments (n = 5), the vasodilator papaverine (10-4 M) did not further vasodilate the vasculature of resting hind limbs, suggesting that the hind limbs in this preparation were fully vasodilated. Papaverine, at constant flow, resulted in a nearly 1.5-fold increase in K+ extraction, a doubling of unidirectional K+ influx, and increases in unidirectional K+ efflux and net K+ efflux. It is concluded that physiological increases in flow rate result in increases in K+ transport in isolated, perfused rat hind-limb skeletal muscle. Furthermore, papaverine appeared to induce an increase in skeletal muscle membrane permeability to K+.Key words: K+ transport, potassium extraction, Na-K ATPase, sarcolemma, plasma flow, vasodilator, VO2.

Effect of blood flow on muscle lactate release studied in perfused rat hindlimb

The influence of blood flow on muscle lactate and H+ release as well as muscle glyconeogenesis was studied in the perfused rat hindlimb. After 2 min of supramaximal stimulation the perfusate flow rate was 7 (F7), 12 (F12), or 18 (F18) ml/min for 30 min. Perfusate samples were drawn frequently and muscle samples were obtained before stimulation, immediately after stimulation, and at 3, 10, and 30 min of recovery from soleus, white gastrocnemius (WG) and red gastrocnemius. During the first 5 min of recovery lactate release was 35-39% lower (P < 0.05) in F7 than in F12 and F18 but with no differences in total release during recovery. In F7 the concentration of lactate was higher (P < 0.05) in soleus after 10 min (18-20%) and in WG after 30 min (63-67%) than in F12 and F18. During the first 2 min of recovery H+ release was 23-34% lower (P < 0.05) in F7 than in F12 and F18. The difference between H+ and lactate release was larger (P < 0.05) in F7 than in F12 and F18 from 3 to 10 min and from 5 to 10 min of recovery, respectively. Muscle glycogen concentrations after 30 min of recovery were independent of flow in each of the muscles. The present data suggest that 1) in the range of blood flow rates from 0.61 to 0.92 ml.min-1.g-1, lactate and H+ release are independent of the flow rate, whereas at a lower flow rate (0.36 ml.min-1.g-1) release of these substances is decreased; 2) low blood flow influences lactate efflux more than H+ release; and 3) muscle glyconeogenesis from lactate is of minor importance.

Effects of flow and contraction on lactate transport in the perfused rat hindlimb

A perfused rat hindlimb preparation was used to assess the effects of perfusate flow and electrical stimulation to mimic exercise on the rates of lactate influx (measured by a dual tracer technique with [3H]mannitol as the extracellular marker) and net lactate production. The same perfused muscle system was also used for assessing the effects of alpha-cyano-4-hydroxycinnamate (CIN, 15 mM), phloretin (0.6 mM), and pyruvate on tracer lactate influx. Unidirectional lactate influx, oxygen uptake (VO2), and net lactate flux were all significantly dependent on perfusate flow rate (all P < 0.05). The hindlimb was in net lactate production at all flow rates studied. Electrical stimulation (60 Hz, 100 ms, 20 V trains at 0.6 min-1) at perfusate lactate concentration of 1 mM significantly increased the hindlimb VO2 from 8.0 +/- 1.1 to 16.0 +/- 2.2 ml.kg-1.min-1 and production of lactate from -69 +/- 31 to -823 +/- 77 nmol.min-1.g-1 (both P < 0.001) but did not affect tracer-measured unidirectional lactate influx (nonstimulated: 235.4 +/- 78.1; stimulated: 235.0 +/- 31.0 nmol.min-1.g-1). At a perfusate flow of 0.55 ml.g-1.min-1 the unidirectional influx of 1 mM lactate was markedly inhibited (90 +/- 5%) by 15 mM CIN. CIN also significantly reduced VO2 from 6.2 +/- 0.16 to 4.45 +/- 0.57 ml.kg-1.min-1 (P < 0.05, n = 5). Phloretin (0.6 mM, n = 3) had no significant effect on lactate influx.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen-dependent modulation of erythropoietin mRNA levels in isolated rat kidneys studied by RNase protection.

Using oligonucleotide primers complementary to conserved regions in the mouse erythropoietin (Epo) gene, a portion of the rat Epo gene was amplified by the polymerase chain reaction to produce a probe suitable for assay of rat Epo mRNA by RNAse protection. The assay, which has sufficient sensitivity to measure to Epo mRNA in unstimulated rat kidneys, was used to demonstrate high amplitude in vitro modulation of Epo mRNA levels in response to changes in perfusate flow rate and oxygen tension in isolated kidneys, thus providing clear evidence that all the necessary events linking changes in oxygen delivery to the modulation of Epo mRNA levels can occur intrarenally.

Sodium excretion during elevation of renal venous pressure: modulation by dDAVP

Studies were performed to determine the effects of elevation of renal venous pressure on sodium excretion by the isolated perfused rat kidney in the presence and absence of a specific V2-receptor agonist, 1-des-amino-8-D-arginine vasopressin (dDAVP), at a concentration (1 ng/ml) expected to have maximal antidiuretic activity but minor vasopressor action. In either the presence or absence of dDAVP, increments in venous pressure led to falls in perfusate flow rate and glomerular filtration rate, which became significant at an imposed pressure greater than or equal to 18.75 mmHg. In the absence of dDAVP, absolute sodium excretion fell as venous pressure increased, and there was a negative correlation between fractional sodium excretion (FENa) and renal venous pressure (RVP) within each experiment and when all data points were combined: FENa = 3.46-0.072RVP (r = -0.608, P less than 0.01). In contrast, in the presence of dDAVP, absolute sodium excretion was unchanged, and in four of five experiments FENa rose as venous pressure increased (in one it remained unchanged). Linear regression analysis of all data points showed a positive correlation between FENa and RVP: FENa = 1.27 + 0.127RVP (r = 0.392, P less than 0.05). The slopes of the two regression lines were significantly different (P less than 0.001). It is postulated that this effect of dDAVP may be mediated via changes in the distal tubular pressure response to elevation of RVP. Such an effect of vasopressin could explain the observation that the response to renal vein constriction in vivo is dependent on volume status.