Fast skeletal muscles of Sprague-Dawley rats [tibialis anterior (TA) and extensor digitorum longus (EDL)] were subjected to ischemia by unilateral ligation of the common iliac artery. In some animals, ischemia was combined with indirect electrical stimulation at 10 Hz either for 3 x 2 h (strenuous activity) or for 7 x 10-min bouts/day (mild activity). After 2 wk, muscle blood flow and fatigue were measured during 5-min isometric supramaximal twitch contractions at 4 Hz. Terminal arteriole diameters were assessed in TA by intravital microscopy at rest and during contractions. Vascular perfusion pressure in the muscles was estimated from measurements in the carotid and saphenous arteries below the site of ligation. Capillary supply was expressed in TA and EDL as capillary-to-fiber ratio on the basis of histochemical staining for capillaries. Strenuous stimulation of ischemic muscles increased their atrophy, failed to restore blood flow, and actually worsened fatigue. In contrast, mild stimulation improved perfusion pressure, increased capillary-to-fiber ratio in the glycolytic part of TA, restored dilatation of terminal arterioles during muscle contractions, and improved blood flow and muscle fatigue so that they were no longer significantly different from control muscles. Thus, an attenuated intermittent protocol may be indicated in the treatment of muscle ischemia.
The effect on the microcirculation of long term administration of the alpha 1 blocker prazosin, which increases peripheral blood flow, was studied in skeletal muscles with differing metabolic profiles. Prazosin (50 mg.litre-1), given ad libitum for 5 weeks in the drinking water, resulted in an increase in capillarity in both fast glycolytic tibialis anterior cortex and slow oxidative soleus muscles of the rat. In tibialis anterior, the velocity of red blood cells through capillaries was increased, a smaller proportion of capillaries had intermittent flow, less time was spent stationary by red blood cells, and hence the calculated volume flow per capillary was increased. In soleus, capillary diameters were larger, but velocity, intermittency and calculated volume flow were unchanged. Therefore, suggested mechanisms for capillary growth induced by prazosin are different for the two muscles studied: increased shear stress and blood/endothelial cell interaction in the case of tibialis anterior, and increased capillary wall tension in the soleus.
Long term limitation of blood supply does not diminish the capillary density or capillary to fibre ratio of anatomically present capillaries in oxidative or glycolytic muscles and does not affect blood flow in capillaries supplying glycolytic fibres appreciably; circulation in capillaries supplying oxidative fibres is impaired.
Sir: Trisomy 9p may be the fourth most common autosomal condition after trisomies 21, 18 and 13. We report a child with trisomy 9p who had a partial Dandy-Walker malformation and maldevelopment of the limbic system. We suggest that these abnormalities may be part of the syndrome but have not been described previously due to the infrequent investigation of cases with MRI.The patient was the ®rst male child of nonconsanguinous Vietnamese parents. At 3 weeks of age the infant was referred to us by a community midwife and dysmorphic features were noted. The clinical features present were typical of trisomy 9p syndrome and included midfacial hypoplasia, hypertelorism, low-set ears, micrognathia, epicanthic folds,``cup-like'' simple ears and downturned corners of the mouth. The infant also had bilateral simian palmar creases and bilateral clinodactyly but nail dysplasia, present in the majority of cases with trisomy 9p, was not found. His karyotype was 46,XY, dup(9)(p12p24). The karyotypes of both parents were normal.At follow up, because of a large anterior fontanelle and developmental delay, brain MRI was performed. This demonstrated the presence of a partial Dandy-Walker malformation with a possible cyst in the cisterna magna (Fig. 1) and abnormal development of the limbic system (Fig. 2).This case demonstrated some of the classical features of trisomy 9p [1]. The ®ndings on the MRI scan of a partial Dandy-Walker malformation and maldevelopment of the limbic system have not been previously reported although a similar picture has been described in trisomy 9 mosaicism [2]. It is likely that previous cases with trisomy 9p would not have had the bene®t of an MRI scan. We would recommend that a brain MRI should become part of the routine investigation of all children with trisomy 9p if they are to be appropriately evaluated.
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