SUMMARYContractile and histochemical properties of soleus (a slow-twitch muscle) and extensor digitorum longus (EDL, a fast-twitch muscle) were studied in mature rats after 3 months of streptozotocin-induced diabetes. Results were compared with age-and weight-matched controls. Diabetes produced profound wasting of fast muscles and particularly of the fast glycolytic (FG) fibres. Slow muscle fibres, both within the mixed EDL and in soleus, were less atrophied. Strength performance of EDL was reduced by diabetes, but maintained in soleus. Diabetes was without effect on the time to peak tension (TTP) and half-relaxation time (HRT) of EDL. However it produced profound slowing of soleus muscles, particularly of the relaxation phase. Part of the slowing effect of diabetes may be related to a histochemically demonstrable loss of fast oxidative glycolytic (FOG) fibres in soleus. Histochemical staining for the oxidative marker succinic dehydrogenase (SDH) revealed marked disruption of reaction product distribution in soleus, indicating an impairment of oxidative capacity.
The effect of synthetic atrial natriuretic factor (ANF) on adrenal steroidogenesis has been studied in primary culture of bovine adrenal cells. ANF-(8-33) produced a potent 40-70% inhibition of angiotensin II-, ACTH-, PGE1-, and forskolin-stimulated secretion of aldosterone production from zona glomerulosa cells with an ED50 of 120 pM. An equipotent inhibitory effect of the natriuretic factor on cortisol production was also observed in cultured zona fasciculata cells. Nicotine-stimulated secretion of catecholamines from medullary cells was only slightly inhibited by the factor at doses above 10 nM. [125I]iodo-ANF-(8-33) binding to glomerulosa membranes displayed an apparent affinity of 100-150 pM for specific receptor sites and was not inhibited by angiotensin II or ACTH. Conversely, the natriuretic factor had no affinity for angiotensin II receptor sites. The results demonstrate that part of the natriuretic effect of this new factor might be due to inhibition of adrenal steroidogenesis by action through a distinct receptor.
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