Selective inhibition by organic mercurials of binding to the beta 1 population of rat renal cortical beta-adrenergic receptors

The beta-adrenergic transduction system in kidneys from young and senescent rats

beta-Adrenoceptor density, ligand affinity, high-affinity agonist binding, basal adenylate cyclase activity and cAMP synthesis upon stimulation with either forskolin, F-, guanine nucleotides (GTP or GppNHp) or isoproterenol in the presence of the nucleotides were studied in membranes prepared from kidneys of young (2-3 month) and old (24-25 month) male Wistar rats. There is a significant (P less than 0.01) 62% increase in beta-receptor density, a significant (P less than 0.05) 115% decrease in ligand affinity, a significant (P less than 0.05) 33% decrease of high-affinity binding sites for (-)-isoproterenol and a significant (P less than 0.01) 151% decrease of the affinity of the high-affinity agonist binding site. Basal adenylate cyclase activity and the activity after stimulation with guanine nucleotides and forskolin were significantly higher in old animals as compared to young (P less than 0.01). Stimulation of the system with isoproterenol in the presence of GTP was more effective in old animals, although the P less than 0.05 level of significance was barely reached. It is suggested that age-dependent changes of the beta-adrenoceptors in rat kidney are similar to those described for lungs: changes at the different levels of the beta-adrenergic transduction chain associated with age are compensatory so as to ensure equal cAMP synthesis for a given agonist stimulation.

Interactions of neurotoxicants with neurotransmitter systems

Many neurotoxic compounds have been shown to interfere with neurotransmission both in vitro and following acute and chronic administration. Various parameters of neurotransmission can be directly affected by neurotoxicants; these include the enzyme(s) synthesizing a neurotransmitter, the release and uptake processes, the enzyme(s) which metabolize the neurotransmitter, the receptors, and post-synaptic events associated with receptor activation. Some neurotoxicants can interfere with neurotransmission indirectly, by interacting for example with energy metabolism, sodium channels or ATPases. Furthermore, measured alterations of any parameter of neurotransmission can be the result of neuronal death, due to a cytotoxic effect of the neurotoxicants. Chemicals which have been shown to alter neurotransmission include solvents (e.g. carbon disulfide), metals and organometals (e.g. lead, mercury, trimethyltin) and pesticides (e.g. organophosphates, pyrethroids, organochlorines, formamidines). An example of the various alterations in neurotransmitter parameters, which can occur following acute or chronic administration, is represented by the organophosphates. Organophosphorus insecticides owe their acute toxicity to inhibition of acetylcholinesterase and accumulation of acetylcholine at cholinergic receptors. Chronic exposure to these compounds results in the development of tolerance to their toxicity which is associated with a decrease in the density of muscarinic and nicotinic receptors in both the central and peripheral nervous system. Other examples of the interactions of neurotoxicants with neurotransmitters are also described.

Localization of beta 1- and beta 2-adrenergic receptors in rat kidney by autoradiography

beta-Adrenergic receptor subtypes were localized and differentiated in rat kidney slices by in vitro autoradiography using the nonselective beta-antagonist [125I]iodocyanopindolol in the presence of the selective agents betaxolol (beta 1) and zinterol (beta 2). [125I]Iodocyanopindolol binding to kidney sections in the presence of these agents could be differentiated into high- and low-affinity components as predicted by the subtype selectivity of the compounds. Autoradiography revealed that: beta-adrenergic receptors were highly concentrated within the renal cortex, especially in glomeruli and juxtaglomerular granule cells, and to tubular sites in the cortex and medulla; [125I]iodocyanopindolol labeling of the juxtaglomeruluar granule cells was abolished at lower concentrations (10(-8) M) of betaxolol than was the labeling of glomeruli (10(-6)--10(-4) M); and zinterol had little effect on labeling of juxtaglomerular granule cells and glomeruli unless high concentrations were used, whereas the tubular labeling in medulla was much more sensitive to incubation with zinterol. These results indicate that beta 1- and beta 2-adrenergic receptor subtypes are differentially distributed within the kidney: beta 1, predominantly contained in juxtaglomerular granule cells and glomeruli, and beta 2, predominantly in medullary tubules. In vitro autoradiography provides a useful means to examine different receptor populations in discrete tissue areas.