In vitro studies with Zaprinast, a weak phosphodiesterase (PDE) 1 and 5 inhibitor (PDE1 IC₅₀ = 6650 nM and PDE5 IC₅₀ = 856 nM), have indicated, in rodent muscle, that enhancing the concentration of intramuscular cGMP may be an approach for enhancing peripheral glucose clearance in diabetes (Young & Leighton, 1998). This effect has previously been attributed to inhibition of PDE5. However, to date this has not been studied in vivo. Using the canine gracilis model (Timmons et al. 1996), where blood flow through the skeletal muscle is controlled by a peristaltic perfusion system, we examined the effects of the potent PDE1/5 inhibitor, UK-114,542 (PDE1 IC₅₀ = 93 nM and PDE5 IC₅₀ = 1.7 nM) on muscle metabolism. Experiments were carried out in accordance with UK legislation. Dogs were anaesthetised (I.V. sodium pentobarbitone 45 ± 1 mg kg^-1 body mass) followed by continuous infusion at 0.10 ± 0.01 mg kg^-1 min^-1 and given a terminal bolus at the end of the experiment. Blood flow was fixed to eliminate any flow-dependent changes in glucose delivery occurring. A systemic free drug concentration of ~75 nM, sufficient to cause total inhibition of PDE5 but minimal inhibition of PDE1, was maintained with a loading dose of 716 mg kg^-1 and continuous infusion at 71 mg kg^-1 h^-1. Measurements were made before and after a 30 min infusion period. Data are means ± S.E.M. (n = 6) with pre-dose data appearing first. All data were statistically analysed using MANOVA followed by a Bonferroni correction. UK-114, 542 evoked a 10 % reduction in mean arterial blood pressure (from 119 ± 4 to 109 ± 3 mmHg, P < 0.05); however, skeletal muscle glucose uptake at rest was not significantly different (4.7 ± 1.8 to 2.6 ± 2.3 µmol min^-1 (100 g wet mass)^-1). The muscle was stimulated to contract, via the obturator nerve (5 Hz, 10 V, 0.2 ms for 2 min) and blood flow was increased 4-fold to meet the oxygen delivery requirements of the task. Peak tension (3.1 ± 0.3 vs. 3.0 ± 0.4 kg (100 g wet mass)^-1; PT), time to PT (30 ± 1.7 vs. 32 ± 0.4 s), oxygen consumption (5.3 ± 0.2 vs. 5.3 ± 0.3 ml min^-1 (100 g wet mass)^-1) and lactate efflux (30.3 ± 9.3 vs. 37.1 ± 10.0 µmol min^-1 (100 g wet mass)^-1) were not significantly altered by UK-114,542. In conclusion and contrary to the in vitro findings of Young et al. (1998) in rodents, inhibition of PDE1/5 did not increase skeletal muscle glucose uptake at rest nor did it alter muscle metabolism during contraction.

The authors would like to acknowledge Paul E. Rajob and David Fairnan for their help as technicians.