Moving the pH Gate of the Kir1.1 Inward Rectifier Channel

Both structural and functional studies suggest that pH gating of the inward rectifier potassium (K) channel, Kir1.1 (ROMK), is mediated by the convergence of 4 hydrophobic leucines (one from each subunit) near the cytoplasmic bundle-crossing of the inner transmembrane helices. We tested this hypothesis by moving the putative leucine gate from the L160-Kir1.1b to other positions along the inner transmembrane helix, and measuring inward current and conductance as functions of internal pH, using the Xenopus oocyte heterologous expression system. Results of these studies indicated that it was possible to replace the putative inward rectifier pH gate at L160-Kir1.1b by either a leucine or methionine at 157-Kir1.1b (G157L-L160G or G157M-L160G). Although both leucine and methionine gated the channel at 157-Kir1.1b, residues of similar hydrophobicity (tyrosine and valine) did not. Hence, hydrophobicity was a necessary but not a sufficient condition for steric gating at 157. This was in contrast to the 160-Kir1.1b locus, where side-chain hydrophobicity was both a necessary and sufficient property for steric gating. Homology models were constructed for all mutants that expressed significant whole-cell currents, using the closed-state coordinates of the prokaryotic inward rectifier, KirBac1.1. Models of mutants that retained pH gating were too narrow at the bundle crossing to permit hydrated K ion permeation in the closed-state. On the other hand, mutants that lost pH gating had ample space at the bundle crossing for hydrated K permeation in the closed-state. These results support our hypothesis that hydrophobic leucines at the cytoplasmic end of the inner transmembrane helices comprise the principal pH gate of Kir1.1, a gate that can be relocated from 160-Kir1.1b to 157-Kir1.1b.     

Identification of the Cationic Amino Acid Transporter (System y+) of the Rat Blood-Brain Barrier

Abstract: Cationic amino acids are transported from blood into brain by a saturable carrier at the blood-brain barrier (BBB). The transport properties of this carrier were examined in the rat using an in situ brain perfusion technique. Influx into brain via this system was found to be sodium independent and followed Michaelis-Men-ten kinetics with half-saturation constants (K_m) of 50–100 μM and maximal transport rates of 22–26 nmol/min/g for L-lysine, L-arginine, and L-ornithine. The kinetic properties matched that of System y⁺, the sodium-independent cationic amino acid transporter, the cDNA for which has been cloned from the mouse. To determine if the cloned receptor is expressed at the BBB, we assayed RNA from rat cerebral microvessels and choroid plexus for the presence of the cloned transporter mRNA by RNase protection. The mRNA was present in both cerebral microvessels and choroid plexus and was enriched in microvessels 38-fold as compared with whole brain. The results indicate that System y⁺ is present at the BBB and that its mRNA is more densely expressed at cerebral microvessels than in whole brain.     

Immunohistochemical properties and spinal connections of pelvic autonomic neurons that innervate the rat prostate gland

Autonomic innervation of the prostate gland supplies the acini, and non-vascular and vascular smooth muscle. The activity of each of these tissues is enhanced by sympathetic outflow, whereas the role of the parasympathetic nervous system in this organ is unclear. In the present study, a range of methods was applied in rats to determine the location of autonomic neurons supplying this gland, the immunohistochemical properties of these neurons, the spinal connections made with the postganglionic pathways and the distribution of various axon types within the gland. Injection of the retrograde tracer, FluoroGold, into the ventral gland visualised neurons within the major pelvic ganglion and sympathetic chain. Fluorescence immunohistochemical studies on the labelled pelvic neurons showed that most were noradrenergic (also containing neuropeptide Y, NPY), the others being non-noradrenergic and containing either vasoactive intestinal peptide (VIP) or NPY. Sympathetic dyelabelled neurons were identified by the presence of varicose nerve terminals stained for synaptophysin on their somata following lesion of sacral inputs. Parasympathetic innervation of dye-labelled neurons was identified by continued innervation after hypogastric nerve lesion. Most noradrenergic prostate-projecting neurons were sympathetic, as were many of the non-noradrenergic VIP neurons. Parasympathetic prostate-projecting neurons were largely non-noradrenergic and contained either VIP or NPY. All substances found in retrogradely labelled somata were located in axons within the prostate gland but had slightly different patterns of distribution. The studies have shown that there are a significant number of non-noradrenergic sympathetic prostate-projecting neurons, which contain VIP.