Protoporphyrins modulate voltage-gated Ca current in AtT-20 pituitary cells.
1. Zinc-protoporphyrin-IX (ZnPP-IX) is an inhibitor of the enzyme heme-oxygenase-2 (HO-2) and consequently has been used to examine the role of carbon monoxide production in neural tissues. We have measured voltage-gated Ca current in AtT-20 pituitary cells using the whole-cell patch-clamp technique and have assessed the effects of extracellularly applied ZnPP-IX and related compounds. 2. Ca currents evoked by depolarizing steps from a holding potential of -90 mV were of the high-threshold, slowly inactivating type. Fifty-six percent of this current was blocked by 10 microM nifedipine and 16% by 3 microM omega-conotoxin with the remainder resistant to both drugs in combination, suggesting that the total Ca current was a mixture of L, N, and possibly P-type conductances. 3. Bath application of ZnPP-IX resulted in an irreversible, dose-dependent attenuation of Ca current. Five micromolar ZnPP-IX produced a 62% reduction of peak current amplitude with no shift in the current-voltage relation, 0.5 microM produced a 19% reduction, and 0.05 microM produced a variable response, either a small transient attenuation or potentiation. 4. The attenuation of Ca current by 5 microM ZnPP-IX could be nearly completely blocked by co-application of superoxide dismutase in the bath (90 U/ml) but not by addition of an inhibitor of cGMP-dependent protein kinase to the internal saline (KT5823, 1 microM). 5. Other inhibitors of heme-oxygenase with similar potency such as tin-protoporphyrin-IX (Sn-PP-IX) and Zn-deuteroporphyrin-bis-glycol (ZnBG) did not attenuate Ca current when applied at 5microM.(ABSTRACT TRUNCATED AT 400 WORDS)
Linden
D J
DJ
Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Narasimhan
K
K
Gurfel
D
D
D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology*; D002248:Carbon Monoxide / Q000378:metabolism; D002460:Cell Line; D005609:Free Radicals; D006162:Guanylate Cyclase / Q000502:physiology; D006419:Heme Oxygenase (Decyclizing) / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D011524:Protoporphyrins / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology*
D015220:Calcium Channels; D005609:Free Radicals; D011524:Protoporphyrins; D002248:Carbon Monoxide; D006419:Heme Oxygenase (Decyclizing); D006162:Guanylate Cyclase; D002118:Calcium