Contractile Function During Acute Ischemia and Reperfusion in Rat Skeletal Muscle

Abstract

When blood flow to skeletal muscle is impeded and oxygen delivery (QO2) is reduced, fatigability increases. The simultaneous measurement of contractile function and microvascular PO2 (PO2mv) permits examination of the relation between PO2mv and the tension-producing capability of muscle. PURPOSE: To measure contractile function, microvascular PO2, and blood flow in the rat extensor digitorum longus muscle (EDL) during an acute ischemic period and subsequent reperfusion. METHODS: The left EDL of female Sprague-Dawley rats (n=5; 301 ± 4 g) was isolated and the distal tendon attached to a load cell connected to a muscle tension analyzer. Microvascular PO2 (PO2mv, phosphorescence quenching) and blood flow (radiolabeled microspheres) were also measured. PO2mv was measured for 40 s before snare-occlusion of the proximal femoral artery. After 70 s of femoral occlusion, muscle contractions were initiated (1 Hz, 6 V). Twitch tension and PO2mv were measured continuously for 10 min of ischemia and after restoration of femoral flow. Blood flow was measured after 4 min of reperfusion. RESULTS: Initial PO2mv averaged 21 ± 2 mmHg, and fell to 6 ± 1 mmHg within 70 s after femoral occlusion. Twitch tension peaked (3.1 ± 0.1 N/g) within 30 s, but decreased over 10 min to 44% of peak (1.4 ± 0.3 N/g). With the restoration of femoral flow, PO2mv increased from 5 ± 1 to 20 ± 4 mmHg with a time constant of 24.4 ± 5.8 s, while twitch tension increased to 76 ± 2% of initial peak tension (2.4 ± 0.1 N/g) with a time constant of 32.0 ± 4.0 s. EDL blood flow at 4 min of reperfusion was 79 ± 8 ml/min/100 g (vs. 18 ± 6 with acute ischemia). CONCLUSION: Although there was marked muscle fatigue during 10 min of femoral artery occlusion, there was sufficient collateral flow to support contractions at a PO2mv of ∼5 mmHg. With restoration of femoral blood flow, PO2mv increased rapidly to near baseline levels and EDL twitch tension increased, reaching 76% of initial peak tension at 4 min. These results demonstrate the ability of skeletal muscle to perform under acute low flow, low PO2 conditions and the rapid rate at which contractile function recovers upon restoration of flow.(These studies were supported by a grant from the KCOM Graduate Program Committee.)