Modulation of Ca2+ currents by various G protein-coupled receptors in sympathetic neurons of male rat pelvic ganglia.
The modulation of voltage-gated calcium (Ca2+) channels by various G protein-coupled receptor pathways was investigated in sympathetic neurons of the male rat major pelvic ganglion (MPG). Standard whole cell patch-clamp recording techniques were used to record Ca2+ currents from acutely dissociated neurons. The activation of muscarinic receptors, which uses a G protein pathway that was not blocked by either pertussis toxin (PTX) or cholera toxin (CTX), inhibited both N-type and L-type Ca2+ channels. The activation of alpha2 noradrenergic receptors with the selective agonist UK14304, which used primarily a PTX-sensitive G protein pathway, inhibited only N-type Ca2+ channels. The activation of vasoactive intestinal polypeptide (VIP) receptors, which used a CTX-sensitive G protein pathway, also inhibited only N-type Ca2+ channels. UK14304 and VIP induced a bell-shaped inhibition of the Ca2+ current with a peak inhibition at around +10 mV and decreasing inhibition at more positive potentials. In contrast, the muscarine-induced Ca2+ current inhibition was not bell shaped and was more prominent at more positive potentials. Furthermore, a large depolarization, which relieved the current inhibition by UK14304 and VIP, did not relieve the inhibition by muscarine. Besides inhibiting the Ca2+ current, UK14304 and VIP also slowed the activation kinetics, an effect not seen with muscarine. Replacing external Ca2+ with Ba2+ and replacing internal ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) with high bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) completely blocked the inhibitory effect of muscarine. However, the inhibitory effects of both UK14304 and VIP were unaffected under these conditions. Surprisingly, the facilitation of the Ca2+ current was eliminated under these strong calcium-buffering conditions. The activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA) increases the amplitude of the Ca2+ current, diminishes facilitation, and reduces the inhibition of this current by UK14304 and VIP. However, PKC activation did not reduce the muscarine-induced Ca2+ current inhibition. In summary, our data suggest that muscarine uses a mechanism different from UK14304 and VIP to modulate the N-type Ca2+ channels in sympathetic neurons of the MPG. Although VIP and UK14304 use different G protein pathways, these two different pathways most likely converge downstream to compete for the same target site on the N-type Ca2+ channels.
Zhu
Y
Y
Laboratory of Signal Transduction, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA.
Yakel
J L
JL
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D000068438:Brimonidine Tartrate; D015220:Calcium Channels / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000502:physiology*; D005728:Ganglia, Sympathetic / Q000187:drug effects / Q000502:physiology*; D007001:Hypogastric Plexus / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009116:Muscarine / Q000494:pharmacology; D018721:Muscarinic Agonists / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D017981:Receptors, Neurotransmitter / Q000187:drug effects / Q000502:physiology*; D017783:Superior Cervical Ganglion / Q000187:drug effects / Q000502:physiology; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D015220:Calcium Channels; D018721:Muscarinic Agonists; D011810:Quinoxalines; D017981:Receptors, Neurotransmitter; D014660:Vasoactive Intestinal Peptide; D000068438:Brimonidine Tartrate; D009116:Muscarine; D019204:GTP-Binding Proteins