Effects of sodium azide, barium ion, d-amphetamine and procaine on inward rectifying potassium channel 6.2 expressed in Xenopus oocytes

Gastrin-releasing peptide acts via postsynaptic BB2 receptors to modulate inward rectifier K+ and TRPV1-like conductances in rat paraventricular thalamic neurons

Gastrin-releasing peptide (GRP) is a bombesin-like peptide with a widespread distribution in mammalian CNS, where it has a role in food intake, circadian rhythm generation, fear memory, itch sensation and sexual behaviour. While it has been established that GRP predominantly excites neurons, details of the membrane mechanism involved in this action remain largely undefined. We used perforated patch clamp recording in acute brain slice preparations to investigate GRP-affected receptors and ionic conductances in neurons of the rat paraventricular thalamic nucleus (PVT). PVT is a component of the midline and intralaminar thalamus that participates in arousal, motivational drives and stress responses, and exhibits a prominence of GRP-like immunoreactive fibres. Exposure of PVT neurons to low nanomolar concentrations of GRP induced sustained TTX-resistant membrane depolarizations that could trigger rhythmic burst discharges or tonic firing. Membrane current analyses in voltage clamp revealed an underlying postsynaptic bombesin type 2 receptor-mediated inward current that resulted from the simultaneous suppression of a Ba(2+)-sensitive inward rectifier K(+) conductance and activation of a non-selective cation conductance with biophysical and pharmacological properties reminiscent of transient receptor potential vanilloid (TRPV) 1. A role for a TRPV1-like conductance was further implied by a significant suppressant influence of a TRPV1 antagonist on GRP-induced membrane depolarization and rhythmic burst or tonic firing. The results provide a detailed picture of the receptor and ionic conductances that are involved in GRP's excitatory action in midline thalamus.

Inward rectifier potassium channels in the HL-1 cardiomyocyte-derived cell line

HL-1 is a line of immortalized cells of cardiomyocyte origin that are a useful complement to native cardiomyocytes in studies of cardiac gene regulation. Several types of ion channel have been identified in these cells, but not the physiologically important inward rectifier K(+) channels. Our aim was to identify and characterize inward rectifier K(+) channels in HL-1 cells. External Ba(2+) (100 µM) inhibited 44 ± 0.05% (mean ± s.e.m., n = 11) of inward current in whole-cell patch-clamp recordings. The reversal potential of the Ba(2+)-sensitive current shifted with external [K(+)] as expected for K(+)-selective channels. The slope conductance of the inward Ba(2+)-sensitive current increased with external [K(+)]. The apparent Kd for Ba(2+) was voltage dependent, ranging from 15 µM at -150  mV to 148 µM at -75  mV in 120  mM external K(+). This current was insensitive to 10 µM glybenclamide. A component of whole-cell current was sensitive to 150 µM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), although it did not correspond to the Ba(2+)-sensitive component. The effect of external 1 mM Cs(+) was similar to that of Ba(2+). Polymerase chain reaction using HL-1 cDNA as template and primers specific for the cardiac inward rectifier K(ir)2.1 produced a fragment of the expected size that was confirmed to be K(ir)2.1 by DNA sequencing. In conclusion, HL-1 cells express a current that is characteristic of cardiac inward rectifier K(+) channels, and express K(ir)2.1 mRNA. This cell line may have use as a system for studying inward rectifier gene regulation in a cardiomyocyte phenotype.