Effect of vasoactive drugs on muscle fibre types and capillaries

The effect of nitroglycerin, nifedipine, diltiazem or propranolol on fibre types and capillaries was studied in extensor digitorum longus (EDL) and soleus (S) muscles of the rat. In EDL muscle nifedipine increased the proportion of type I fibres (7.9% +/- 1.7 vs. 4.7% +/- 2.7). Nitroglycerin and dialtiazem decreased IIB fibres (40.7% +/- 10.6 and 37.3% +/- 14.6 respectively vs. 52.3% +/- 9.4). Propranolol increased IIB fibres to 66.3% +/- 8.1, while reducing IIA fibres (25.3% +/- 6.6 vs. 42.2% +/- 6.9). No changes in fibre type proportion were found in S muscle. Capillary density was increased in EDL by nitroglycerin (965 +/- 171 vs. 818 +/- 98 cap/mm2). Propranolol had a dual effect on this parameter, decreasing it in EDL to 570 +/- 85 and augmenting it in S (754 +/- 117 vs. 601 +/- 121). No change was found in capillary to fibre ratio with any of the drugs either in EDL or S muscles. In EDL all the drugs except propranolol, which had the opposite effect, decreased the area of IIA fibre per capillary around 20%; this effect can be interpreted as a reduction of diffusion distance from blood to fibre.