The effect of EGTA and Ca++ in regulation of the brain Na/K-ATPase by noradrenaline

Some Kinetic Features of Na,K-ATPase and Sensitivity to Noradrenaline

A comparative kinetic analysis of albino rat brain synaptic and kidney plasma membrane fraction Na,K-ATPase was performed to comprehend the different levels of sensitivity of these fractions to the neurotransmitter noradrenaline. Noradrenaline (NA) inhibits the rat brain synaptic membrane Na,K-ATPase, changes the stoichiometry of Na⁺ and K⁺ and shifts the enzyme system from an MgATP to an Mg²⁺ dependent cycle. While the kidney plasma membrane fraction Na,K-ATPase is not sensitive to noradrenaline. To investigate the mechanism underlying this difference, we studied enzyme velocity dependence on the concentration of Mg²⁺. The 1/V = f(Mg²⁺) function has shown different kinetic features for the synaptic and kidney plasma membrane Na,K-ATPase. With the addition of ethylene glycol tetraacetic acid (EGTA) to the reaction medium the geometric form of 1/V = f(Mg²⁺) function is affected differently. We thereafter measured the essential activator number for Na⁺ and K⁺ with, in excess Mg²⁺. The results of these experiments reveal that, contrary to the synaptic membrane Na,K-ATPase, the kidney plasma membrane fraction Na,K-ATPase does not possess an Mg²⁺ dependent cycle and noradrenaline exhibits different modulatory effects on the enzyme system.

Inhibitory role of monovalent ions on rat brain cortex adenylyl cyclase activity

Adenylyl cyclases, comprise of a large family of enzymes that catalyze synthesis of the cyclic AMP from ATP. The aim of our study was to determine the effect of monovalent ions on both basal, stimulated adenylate cyclase EC 4.6.1.1 (AC) activity and C unit of AC and on GTPase active G-protein in the synaptic membranes of rat brain cortex. The effect of ion concentration from 30 to 200 mM (1 mM MgCl2) showed dose-dependent and significant inhibition of the basal AC activity, stimulated and unstimulated C unit activity. Stimulation of AC with 5 μM GTPγS in the presence of 50-200 mM of tested salts showed inhibitory effect on the AC activity. From our results it could be postulated that the investigated monovalent ions exert inhibitory effect on the AC complex activity by affecting the intermolecular interaction of the activated α subunit of G/F protein and the C unit of AC complex an inhibitory influence of tested monovalent ions on these molecular interaction.

Effects of Pb2+ ions on Na+ transport in the isolated skin of the toad Pleurodema thaul

The effects induced by lead ions on the short-circuit current (SCC) and on the potential difference (V) of the toad Pleurodema thaul skin were investigated. Pb2+ applied to the outer (mucosal) surface increased SCC and V and when applied to the inner (serosal) surface decreased both parameters. The stimulatory effect, but not the inhibitory action, was reversible after washout of the metal ion. The amiloride test showed that the increase was due principally to stimulation of the driving potential for Na+ (V-E(Na+)) and that inhibition was accompanied by a reduction in the V-E(Na+) and also by a significant decrease in skin resistance indicating possible disruption of membrane and/or cell integrity. The effect of noradrenaline was increased by outer and decreased by inner administration of Pb2+. The results suggest that mucosal Pb2+ activates toad skin ion transport by stimulating the V-E(Na+) and that serosal Pb2+, with easier access to membrane and cellular constituents, inactivates this mechanism, revealing greater toxicity when applied to the inner surface of the skin.