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SUMMARY
The effects of the pineal gland on the ultrastructure of pituitary gonadotrophs have been studied. The results of luteinizing hormone (LH) estimations in the same rats have already been reported (Clementi, De Virgiliis, Fraschini & Mess, 1966; Fraschini, Mess & Martini, 1968). It was found that pinealectomy in normal animals greatly increases the pituitary content of gonadotrophins without significantly changing the ultrastructure of gonadotrophs.
Pinealectomy in castrated animals did not induce an increase in pituitary gonadotrophin stores above the level found after castration alone; the ultrastructure of the 'castration cells' was also unchanged.
Implants of melatonin or of pineal tissue into the median eminence or the reticular formation of the midbrain of castrated animals reduced the pituitary content of LH and, morphologically, the gonadotrophs reverted to a nearly normal state with a reduction in volume of the cisternae of the endoplasmic reticulum.
The relationship between gonadotrophin content and the ultrastructural appearance of the gonadotrophs is discussed.
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SUMMARY
The neurohaemal part of the median eminence of the rat hypothalamus is characterized by numerous nerve terminals which end near a rich network of fenestrated capillaries. An attempt was made to isolate different types of nerve terminals by means of sucrose density-gradient centrifugation. The subcellular fractions obtained were assayed for dopamine, noradrenaline and 5-hydroxytryptamine. In addition FSH- and GH-releasing activities were determined. A sample of each fraction obtained was taken for electron microscopical observations.
Dopamine, noradrenaline, 5-hydroxytryptamine, GH- and FSH-releasing factors were present in higher concentration in the nerve endings. A further fractionation showed that noradrenaline was present in the lightest synaptosomal band, dopamine in the middle one, and 5-hydroxytryptamine in the heaviest. GH-RF and FSH-RF were recovered mainly from the band containing dopamine. The relevance of this localization to the physiological role of the median eminence is discussed.
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Abstract
Calcium ion entry through voltage-operated calcium channels is a crucial step in the coupling of β cell depolarization with insulin secretion. Various calcium channel subtypes have been shown to be coexpressed in single neurons and endocrine cells. Using the patch-clamp technique, we investigated the biophysical and pharmacological properties of calcium channels in freshly dispersed human pancreatic β cells.
Both low and high voltage activated currents were expressed, the two current types being easily distinguishable on the basis of biophysical criteria. The high voltage activated currents were not homogeneous: one component was affected by the dihydropyridine antagonist nitrendipine and the agonist Bay-K-8644; the other was insensitive to both dihydropyridines and ω-conotoxin GVIA. In line with this pharmacology, nitrendipine reduced and Bay-K-8644 increased glucose-induced insulin secretion from perifused human islets, whereas ω-conotoxin GVIA had no effect. However, about 20% of the glucose-induced insulin release was found to be resistant to high nitrendipine concentrations.
These data show that human pancreatic β cells express heterogeneous voltage-operated calcium channels, only one of which is dihydropyridine-sensitive (L type). The L type channels are clearly involved in the control of insulin secretion, but our data suggest that dihydropyridine- and ω-conotoxin GVIA-insensitive channels may also play a role in the stimulus-secretion coupling of human β cells.
Journal of Endocrinology (1996) 150, 195–203
