Mercuric chloride inhibition of vasopressin release from the isolated neurointermediate lobe of the rat pituitary

The endocrine disruptive effects of mercury

Mercury, identified thousands of years ago is one of the oldest toxicants known. The endocrine disruptive effects of mercury have recently become one of the major public concerns. In this report, the adverse effects of mercury on the hypothalamus, pituitary, thyroid, adrenal gland, and gonads (testis and ovary) in laboratory animals as well as in humans are reviewed. The effects of both environmental and occupational exposures to organic, inorganic, or metallic mercury are explained. There is sufficient evidence from animal studies supporting the disruptive effects of mercurials on the functions of the thyroid, adrenal, ovary, and testis, although several factors make it difficult to extrapolate the animal data to the human situation. However, the human studies performed so far, which focused mainly on serum hormone levels, failed to provide any conclusive data to confirm the findings from the animal studies. Therefore, further well-designed epidemiological studies are urgently needed. The possible mechanisms of the toxic effects are also discussed. The broad enzyme inhibition and the influence on the combining of hormones by their receptors, which seem due to its avid binding to sulphydryl, may account for the primary mechanism. The interference with intracellular calcium metabolism, and peroxidation may also be involved.

The effects of mercuric chloride on calmodulin-mediated Ca2+ transport in rat brain

We have shown previously that mercuric chloride (HgCl2) inhibits in vitro vasopressin release from the isolated rat neurohypophysis with maximum inhibition occurring with 0.5 mM HgCl2. Associated with the inhibition of hormone release is an increase in 45Ca+2 uptake, an increase in cytosolic 45Ca+2, and a reduction of 45Ca+2 accumulation by mitochondria in the intact gland. In the present series of studies, the effect of HgCl2 on calmodulin (CM) function in neural tissue preparations is reported. Mercuric chloride (0.5 mM) reduced 45Ca+2 binding to CM purified from bovine neurohypophyses by 20% and inhibited endogenous CM-stimulated Ca,Mg-ATPase activity from rat brain mitochondria in a dose-dependent fashion. Ca,Mg-ATPase activity was inhibited by 50 and 80% with 0.5 and 5.0 mM HgCl2, respectively. CM-stimulation of Ca,Mg-ATPase activity was inhibited by calmidazolium (CMZ) with maximal inhibition seen with 0.1 mM CMZ. Reversibility of the HgCl2 interaction with CM was demonstrated using CM-stimulated phosphodiesterase (PDEase) activity from rat brain. HgCl2 inhibited both basal and CM-stimulated PDEase activity in a dose-dependent manner with maximum inhibition occurring with 1.0 mM HgCl2. Preexposure of CM to an inhibitory concentration (1.0 mM) of HgCl2 resulted in no loss of stimulatory PDEase enzyme activity. From these results, we conclude that HgCl2 reversibly interferes with 45Ca+2 binding to CM and also inhibits CM-regulated Ca+2 pumping enzyme systems in the neurohypophysis. The inhibition of vasopressin release from the intact gland in the presence of HgCl2 thus, may be associated with a disruption of calcium in the neurohypophysis.