Methylmercury blocks N- and L-type Ca++ channels in nerve growth factor-differentiated pheochromocytoma (PC12) cells.
Effects of methylmercury (MeHg) on whole-cell Ba++ currents in rat pheochromocytoma (PC12) cells were examined. Based on biophysical characteristics and sensitivity to omega-conotoxin GVIA and dihydropyridine agonists and antagonists, voltage-activated Ba++ currents (IBa) in PC12 cells were mediated by N- and L-type Ca++ channels. Addition of MeHg (10 microM) to the extracellular solution caused a rapid and complete block of current carried by 20 mM Ba++. The rate of block of IBa by MeHg increased in a concentration-dependent manner between 1 and 20 microM. Increasing the frequency of stimulation from 0.1 to 0.4 Hz facilitated block of IBa by MeHg. A 2-min application of 10 microM MeHg in the absence of stimulation also reduced IBa by approximately 80%. Thus, block of IBa by MeHg is not state-dependent. Additionally, MeHg blocked IBa when the membrane holding potential was -40, -70 and -90 mV, indicating that both N- and L-type Ca++ channels are blocked by MeHg. Block of IBa by MeHg was voltage-dependent at a membrane holding potential of -40 mV, but not at holding potentials of -70 and -90 mV. Decreasing the extracellular concentration of Ba++ ([Ba++]e) from 20 mM to 10 mM increased the magnitude of block by MeHg from 45.6 to 77.3%. Increasing [Ba++]e to 30 mM caused no further antagonism of block. Block of IBa by MeHg was not reversed by washing with MeHg-free solution. The ionic permeability of PC12 cell Ca++ channels was Ca++ = Sr++ greater than Ba++. In the presence of MeHg, all three divalent cations were equally permeant.(ABSTRACT TRUNCATED AT 250 WORDS)
Shafer
T J
TJ
Department of Pharmacology and Toxicology, Michigan State University, East Lansing.
Atchison
W D
WD
D000310:Adrenal Gland Neoplasms / Q000378:metabolism*; D000818:Animals; D001464:Barium / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D002463:Cell Membrane Permeability / Q000187:drug effects; D004558:Electric Stimulation; D008564:Membrane Potentials / Q000187:drug effects; D008767:Methylmercury Compounds / Q000494:pharmacology*; D009414:Nerve Growth Factors / Q000378:metabolism*; D010673:Pheochromocytoma / Q000378:metabolism*; D051381:Rats; D014407:Tumor Cells, Cultured
D015220:Calcium Channels; D008767:Methylmercury Compounds; D009414:Nerve Growth Factors; D001464:Barium