Capillary proliferation and microvessel diameters were studied in rat ankle flexors subjected to chronic electrical stimulation by implanted electrodes (10 Hz, 0.3 ms pulse width, up to 6 V, 8 h day-1) for 2 or 7 days with or without concurrent indomethacin treatment ( approximately 2 mg day-1 in drinking water) to study the role of prostaglandins in the microcirculation in relation to capillary growth. 2. Diameters of terminal arterioles, capillaries and confluent venules were measured in epi-illuminated muscles, together with capillary red cell velocity, to evaluate whether changes in capillary pressure and/or shear stress participate in capillary growth via release of prostaglandins. 3. Cell proliferation was detected following bromodeoxyuridine (BrdU) incorporation and immuno-staining of frozen sections. Labelling was assessed as the percentage of all interstitial nuclei (Haematoxylin-stained) that were BrdU positive. By comparison with serial sections stained for alkaline phosphatase, from which the capillary-to-fibre ratio (C:F) was obtained, labelling was derived for nuclei colocalised either to capillaries or to other non-capillary interstitial cells. 4. C:F increased to 1.89 +/- 0.06 from 1.47 +/- 0.04 in controls only after 7 days stimulation; indomethacin reduced this to 1.55 +/- 0.07. Capillary labelling increased from 2.9 +/- 0.5 % in controls to 11.3 +/- 2.2 % after 2 days stimulation and 10.6 +/- 0.8 % after 7 days. The increase was attenuated by indomethacin at both time points (to 5.8 +/- 1.6 % and 4.2 +/- 0.5 %, respectively). 5. Non-capillary interstitial labelling (2.0 +/- 0.4 % in controls) increased to 9.5 +/- 2.7 % after 2 days stimulation and was back to normal after 7 days (3.2 +/- 0.7 %). Indomethacin depressed the increase at 2 days to 4.0 +/- 1.3 % and had no effect at 7 days (2.9 +/- 0.13 %). Labelling in sham-operated rats with or without indomethacin or in vehicle-treated animals was no different from controls. 6. Arteriolar and venular diameters were increased by 2 days of stimulation but unchanged after 7 days. Indomethacin increased diameters of arterioles after 2 days and venules after 7 days in sham-operated animals, but had no effect on diameters of either vessel type in stimulated muscles. 7. Capillary diameters did not change during acute muscle contractions whereas red cell velocity did. Calculated shear stress in capillaries was thereby increased by 75 %. 8. Thus during chronic electrical stimulation both capillary growth and the cell proliferation that precedes it were attenuated by indomethacin. Transient stimulation-induced increases in arteriolar and venular diameters, which were unaffected by indomethacin, do not implicate increased capillary pressure as a factor in prostaglandin release and capillary growth. Estimations of increases in capillary shear stress during muscle contractions and of a 45 % higher value even at rest after chronic stimulation for 7 days suggest that shear stress is a more likely stimulus for prostaglandin release in chronically stimulated muscles.
