Selective G-Protein Regulation of Neuronal Calcium Channels

Abstract

We examined the properties and regulation of Ca channels resulting from the expression of human ␣ 1B and ␣ 1E subunits stably expressed in HEK293 cells. The ancillary subunits ␤ 1B and ␣ 2 /␦ were also stably expressed in these cell lines. Ca currents in ␣ 1B -expressing cells had the properties of N-type currents. Ca currents in cells expressing ␣ 1E exhibited a novel profile that was similar to the properties of the "R type" Ca current. Introduction of GTP-␥-S into ␣ 1B cells greatly enhanced the extent of prepulse facilitation of the Ca current, whereas it had only a very small effect in ␣ 1E -expressing cells. Activation of somatostatin receptors endogenous to HEK293 cells or opioid receptors, expressed in the cells after transfection, inhibited Ca currents in ␣ 1B -expressing cells. This inhibition was blocked by pertussis toxin and was partially relieved by a depolarizing prepulse. In contrast, no inhibitory effects were noted in cells expressing ␣ 1E channels under the same circumstances. HEK293 cells normally contained G-proteins from all of the four major families. Inhibition of Ca currents by agonists in ␣ 1B -expressing cells was enhanced slightly by the cotransfection of several G-protein ␣ subunits. agonists, however, had no effect in ␣ 1E -containing cells, even after overexpression of different G-protein ␣-subunits. In summary, these results demonstrate that there is a large difference in the susceptibility of ␣ 1B -and ␣ 1E -based Ca channels to regulation by G-proteins. This is so despite the fact that the two types of Ca channels show substantial similarities in their primary sequences.

Somatostatin Inhibits Excitatory Transmission at Rat Hippocampal Synapses via Presynaptic Receptors

“…6 A, inset). Hence, the somatostatin-induced inhibition displayed the characteristics of a G-protein-mediated voltage-dependent modulation of Ca 2ϩ channels (Zhang et al, 1996), which has been described for somatostatin in other preparations (Ishibashi and Akaike, 1995;Toth et al, 1996;Zhang et al, 1996).…”

“…At hippocampal synapses, N-and Q-type Ca 2ϩ channels contribute to synaptic transmission (Wheeler et al, 1994). N-type channels provide ϳ50% of the Ca 2ϩ entry necessary for transmission (Wheeler et al, 1994(Wheeler et al, , 1996, and these channels are preferentially inhibited by somatostatin receptors (Toth et al, 1996;Zhang et al, 1996). Hence, under physiological conditions in which postsynaptic responses are proportional to the fourth power of the presynaptic Ca 2ϩ current (Borst and Sakmann, 1996), reduction of currents through one type of Ca 2ϩ channel may be sufficient to cause inhibition of transmitter release.…”

“…If, however, the contribution of one type of Ca 2ϩ channel becomes redundant, for example, by broadening of action potentials (Wheeler et al, 1996) during neuronal bursting (see Kaczmarek and Levitan, 1987), a modulation of this channel will be ineffective. Furthermore, the inhibition of all types of neuronal Ca 2ϩ currents via G-proteins is largely attenuated or abolished when depolarizations are applied at short intervals (Ͻ50msec) (Grassi and Lux, 1989;Toth et al, 1996;Zhang et al, 1996). Hence, high-frequency firing may also hinder the receptor-mediated modulation of Ca 2ϩ channels.…”

Facilitation of rabbit α1B calcium channels: involvement of endogenous Gβγ subunits

“…No staining was observed in cells transfected with pMT2 vector alone or cells stained using á1B pre-immune serum. & Miller, 1996). Dialysis with GDPâS reduced control facilitation at −30 mV, indicating that rather than being an intrinsic property of the á1B calcium channel, as may be the case for á1C (Scuptoreanu et al 1993a), it may be a result of tonic modulation by endogenous free Gâã subunits, whose level could be influenced by the endogenous GTP concentration, and possibly by tonic activation of endogenous receptors (Koch et al 1994).…”

G protein inhibition of CaV2 calcium channels

“…[96][97][98] However, the majority of studies have focused on Nand P/Q-type channels in part because these are prominently inhibited and in part because they underlie the Ca 2+ entry that triggers neurotransmitter release at most synapses and some neuroendocrine cells. Although very similar, there are subtle differences in the inhibition of Ca V 2.1 and Ca V 2.2 channels that might have important physiological consequences.…”