ISSN:
1432-2013
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract Under conditions of low intracellular [Mg2+] ([Mg2+]i), achieved by dialysis with pipette solutions containing ethylenediamine tetraacetic acid (EDTA), 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) and adenosine triphosphate (ATP) as chelator, calcium currents through the L-type calcium channels (I Ca) were increased in frog ventricular myocytes. Total suppression of phosphorylation by depleting the cell of ATP with a cocktail of β,γ-methyleneadenosine 5′-triphosphate (AMP-PCP) 2-deoxyglucose and carboxylcyanide-M-chlorophenylhydrazone (CCCP) did not inhibit the increase in I Ca in the Mg2+-deficient medium. Thus, the involvement of phosphorylation process in the increase in I Ca was not likely. Effective suppression of this enhancement of I Ca was achieved by the application of guanosine triphosphate (GTP). From the dose-response curve for GTP, the GTP concentration required for half-maximal inhibition (IC50) was estimated to be 4.0 μM at pMg 6. This GTP-induced suppression of I Ca is not due to the guanine nucleotide binding protein (G-protein) cascade, because both activators and inhibitors of G-protein, which are structural analogues of GTP, suppressed I Ca similarly. Treatment with pertussis toxin (PTX) did not affect the inhibitory action of Mg2+ and GTP on I Ca. GTP is therefore assumed to bind directly to the Ca2+ channel. Interaction of Mg2+ and GTP with the Ca2+ channel activated in the Mg2+-deficient medium was examined by comparing the dose/response curves for GTP at two different [Mg2+]. The IC50 for GTP suppression was estimated to be 5.7 μM at pMg 6 and 6.9 μM at pMg 5. The results suggest strongly that Mg2+ and GTP independently bind and control Ca2+ channels.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s004240050155
