A model for the stimulation of taste receptor cells by salt.

A taste cell mucosal surface is regarded as a planar region containing bound anionic sites and openings to ionic channels. It is assumed that the bulk aqueous properties of the exterior phase are not continuous with the surface but terminate at a plane near the surface. The region between the (Stern) plane and the membrane is regarded as having a lower dielectric constant than bulk water. This fact admits the possibility of ion pair formation between fixed sites and mobile cations. Mobile ion pairs entering the region may also bind to a fixed anionic site. Thus, it is assumed that mobile cations and ion pairs are potential determining species at the surface. Binding cations neutralizes surface charges, whereas binding mobile ion pairs does not. This competition accounts for the observed anion effect on stimulation of tast receptors by sodium salts. The potential profile is constructed by superimposing the phase boundary potentials with an ionic diffusion potential across the membrane. The model accounts for the anion effect on receptor potential, pH effects, the reversal of polarity when cells are treated with FeCl3, and the so-called "water reponse," depolarization of the taste cell upon dilution of the stimulant solution below a critical lower limit. The proposed model does not require both bound cationic and anionic receptors, and further suggests that limited access to a Stern-like region continuous with membrane channels may generally serve to control transport of ions.     

Ethanol modulates the VR-1 variant amiloride-insensitive salt taste receptor. I. Effect on TRC volume and Na+ flux

The effect of ethanol on the amiloride- and benzamil (Bz)-insensitive salt taste receptor was investigated by the measurement of intracellular Na(+) activity ([Na(+)](i)) in polarized rat fungiform taste receptor cells (TRCs) using fluorescence imaging and by chorda tympani (CT) taste nerve recordings. CT responses were monitored during lingual stimulation with ethanol solutions containing NaCl or KCl. CT responses were recorded in the presence of Bz (a specific blocker of the epithelial Na(+) channel [ENaC]) or the vanilloid receptor-1 (VR-1) antagonists capsazepine or SB-366791, which also block the Bz-insensitive salt taste receptor, a VR-1 variant. CT responses were recorded at 23 degrees C or 42 degrees C (a temperature at which the VR-1 variant salt taste receptor activity is maximally enhanced). In the absence of permeable cations, ethanol induced a transient decrease in TRC volume, and stimulating the tongue with ethanol solutions without added salt elicited only transient phasic CT responses that were insensitive to elevated temperature or SB-366791. Preshrinking TRCs in vivo with hypertonic mannitol (0.5 M) attenuated the magnitude of the phasic CT response, indicating that in the absence of mineral salts, transient phasic CT responses are related to the ethanol-induced osmotic shrinkage of TRCs. In the presence of mineral salts, ethanol increased the Bz-insensitive apical cation flux in TRCs without a change in cell volume, increased transepithelial electrical resistance across the tongue, and elicited CT responses that were similar to salt responses, consisting of both a transient phasic component and a sustained tonic component. Ethanol increased the Bz-insensitive NaCl CT response. This effect was further enhanced by elevating the temperature from 23 degrees C to 42 degrees C, and was blocked by SB-366791. We conclude that in the presence of mineral salts, ethanol modulates the Bz-insensitive VR-1 variant salt taste receptor.     

An Approach to the Stepwise Management of Severe Mitral Regurgitation with Optimal Cardiac Pacemaker Function

Right ventricular apical pacing may cause or worsen mitral regurgitation (MR). Potential mechanisms for this adverse sequelae include intraventricular dyssynchrony, altered papillary muscle function, pacing-induced cardiomyopathy with left ventricular dilation, and annular dilation. In contrast, biventricular (BiV) pacing may improve MR presumably by opposing the negative effects. Whether or not left ventricular lead location is important in treating mitral regurgitation in patients with pacemakers is unknown.We report a case of severe MR and left ventricular (LV) systolic failure in a patient with right ventricular pacing. Multiple potential etiologies for the worsening valve function were noted, and a stepwise iterative optimizing scheme that included basal lateral LV pacing improved mitral valve function and ameliorated heart failure symptoms.     

Intramural conduction system gradients and electrogram regularity during ventricular fibrillation

Introduction

The His-Purkinje system has been shown to harbor triggers for ventricular fibrillation (VF) initiation. However, the substrate responsible for VF maintenance remains elusive. We hypothesized that standard, electrode-based, point-to-point mapping would yield meaningful insight into site-specific patterns and organization which may shed light on the critical substrate for maintenance of VF.

Direct measurement of translingual epithelial NaCl and KCl currents during the chorda tympani taste response.

We have measured the NaCl or KCl currents under voltage clamp across the dorsal lingual epithelium of the rat and simultaneously the response of the taste nerves. Under short-circuit conditions a NaCl stimulus evoked an inward current (first current) that coincided with excitation of the chorda tympani. This was followed by a slower inward current (second current) that matched the kinetics of taste nerve adaptation. The peak first current and the coincident neural response satisfied the same saturating NaCl concentration dependence. Both first and second currents were partially blocked by amiloride as were the phasic and tonic components of the neural response. The NaCl-evoked second current was completely blocked by ouabain. Investigation of the NaCl-evoked current and the neural response over a range of clamped voltages showed that inward negative potentials enhanced the inward current and the neural response to 0.3 M NaCl. Sufficiently high inward positive potentials reversed the current, and made the neural response independent of further changes in voltage. Therefore, one of the NaCl taste transduction mechanisms is voltage dependent while the other is voltage independent. A KCl stimulus also evoked an inward short-circuit current, but this and the neural response were not amiloride-sensitive. The data indicate that neural adaptation to a NaCl stimulus, but not a KCl stimulus, is mediated by cell Na/K pumps. A model is proposed in which the connection between the NaCl-evoked second current and cell repolarization is demonstrated.     

Complexity of the mechanisms of initiation and maintenance of DNA damage-induced G2-phase arrest and subsequent G1-phase arrest: TP53-dependent and TP53-independent roles

Through a detailed study of cell cycle progression, protein expression, and kinase activity in gamma-irradiated synchronized cultures of human skin fibroblasts, distinct mechanisms of initiation and maintenance of G2-phase and subsequent G1-phase arrests have been elucidated. Normal and E6-expressing fibroblasts were used to examine the role of TP53 in these processes. While G2 arrest is correlated with decreased cyclin B1/CDC2 kinase activity, the mechanisms associated with initiation and maintenance of the arrest are quite different. Initiation of the transient arrest is TP53-independent and is due to inhibitory phosphorylation of CDC2 at Tyr15. Maintenance of the G2 arrest is dependent on TP53 and is due to decreased levels of cyclin B1 mRNA and a corresponding decline in cyclin B1 protein level. After transiently arresting in G2 phase, normal cells chronically arrest in the subsequent G1 phase while E6-expressing cells continue to cycle. The initiation of this TP53-dependent G1-phase arrest occurs despite the presence of substantial levels of cyclin D1/CDK4 and cyclin E/CDK2 kinase activities, hyperphosphoryated RB, and active E2F1. CDKN1A (also known as p21(WAF1/CIP1)) levels remain elevated during this period. Furthermore, CDKN1A-dependent inhibition of PCNA activity does not appear to be the mechanism for this early G1 arrest. Thus the inhibition of entry of irradiated cells into S phase does not appear to be related to DNA-bound PCNA complexed to CDKN1A. The mechanism of chronic G1 arrest involves the down-regulation of specific proteins with a resultant loss of cyclin E/CDK2 kinase activity.     

Structure of integrin, a glycoprotein involved in the transmembrane linkage between fibronectin and actin

We describe the isolation, characterization, and sequence of cDNA clones encoding one subunit of the complex of membrane glycoproteins that forms part of the transmembrane connection between the extracellular matrix and the cytoskeleton. The cDNA sequence encodes a polypeptide of 89 kd that has features strongly suggesting the presence of a large N-terminal extracellular domain, a single transmembrane segment, and a small C-terminal cytoplasmic domain. The extracellular domain contains a threefold repeat of a novel 40 residue cysteine-rich segment, and the cytoplasmic domain contains a tyrosine residue that is a potential site for phosphorylation by tyrosine kinases. We propose the name integrin for this protein complex to denote its role as an integral membrane complex involved in the transmembrane association between the extracellular matrix and the cytoskeleton.