Voltage-dependent ionic currents in taste receptor cells of the larval tiger salamander

Voltage-dependent membrane currents of cells dissociated from tongues of larval tiger salamanders (Ambystoma tigrinum) were studied using whole-cell and single-channel patch-clamp techniques. Nongustatory epithelial cells displayed only passive membrane properties. Cells dissociated from taste buds, presumed to be gustatory receptor cells, generated both inward and outward currents in response to depolarizing voltage steps from a holding potential of -60 or -80 mV. Almost all taste cells displayed a transient inward current that activated at -30 mV, reached a peak between 0 and +10 mV and rapidly inactivated. This inward current was blocked by tetrodotoxin (TTX) or by substitution of choline for Na+ in the bath solution, indicating that it was a Na+ current. Approximately 60% of the taste cells also displayed a sustained inward current which activated slowly at about -30 mV and reached a peak at 0 to +10 mV. The amplitude of the slow inward current was larger when Ca2+ was replaced by Ba2+ and it was blocked by bath applied CO2+, indicating it was a Ca2+ current. Delayed outward K+ currents were observed in all taste cells although in about 10% of the cells, they were small and activated only at voltages more depolarized than +10 mV. Normally, K+ currents activated at -40 mV and usually showed some inactivation during a 25-ms voltage step. The inactivating component of outward current was not observed at holding potentials more depolarized -40 mV. The outward currents were blocked by tetraethylammonium chloride (TEA) and BaCl2 in the bath or by substitution of Cs+ for K+ in the pipette solution. Both transient and noninactivating components of outward current were partially suppressed by CO2+, suggesting the presence of a Ca2(+)-activated K+ current component. Single-channel currents were recorded in cell-attached and outside-out patches of taste cell membranes. Two types of K+ channels were partially characterized, one having a mean unitary conductance of 21 pS, and the other, a conductance of 148 pS. These experiments demonstrate that tiger salamander taste cells have a variety of voltage- and ion-dependent currents including Na+ currents, Ca2+ currents and three types of K+ currents. One or more of these conductances may be modulated either directly by taste stimuli or indirectly by stimulus-regulated second messenger systems to give rise to stimulus-activated receptor potentials. Others may play a role in modulation of neurotransmitter release at synapses with taste nerve fibers.(ABSTRACT TRUNCATED AT 400 WORDS)     

Immunochemical evidence that myosin I heavy chain-like protein is identical to the 110-kilodalton brush-border protein

In a previous study, we identified a new mammalian myosin heavy chain, termed myosin I heavy chain-like protein (MIHC), by molecular cloning of a bovine intestinal cDNA clone. In this investigation, we examined the relationship between MIHC and the 110-kDa intestinal brush-border protein, which possesses a myosin-like ATPase activity. We raised antibodies against a chemically synthesized oligopeptide representing a part of the MIHC sequence. These antibodies reacted specifically in immunoblots with the 110-kDa protein in both purified 110-kDa protein-calmodulin complex and crude microvillar protein extracts. Staining of tissue sections with these antibodies was specifically localized to the brush-border microvilli of small intestines, indicating an identical cellular localization for both MIHC and the 110-kDa protein. Furthermore, analysis of the MIHC sequence revealed two putative calmodulin-binding sites, which is consistent with the fact that the 110-kDa protein forms a complex with calmodulin. These results strongly support the conclusion that MIHC is identical to the 110-kDa protein and suggest that not only the conventional myosin system but also the MIHC (110-kDa protein)-calmodulin complex may play an important role in ATP-dependent and Ca2+-induced brush-border contraction.     

Molecular cloning and characterization of the platelet-activating factor receptor gene expressed in the human heart.

PAF decreases cardiac contractility and blood pressure. To characterize the cardiac PAF receptor, we screened a human ventricular cDNA library in a low stringency condition, using a PCR product derived from guinea pig lung PAF receptor as a probe. Four clones were obtained and named HV1-4. In Xenopus oocytes injected with cRNA derived from HV3 or 4 but not from HV1 or 2, PAF elicited a Ca(2+)-activated Cl- current. HV3 and HV4 were duplicate clones, encoding a 342 amino-acid polypeptide which was identical to that of the human leukocyte PAF receptor. However, a portion of the 5' untranslated region of HV3 (or 4) was different from that of the leukocyte receptor cDNA. Northern blotting of human ventricles and atria using the HV3 insert showed a single band of approximately 4 kb. These results suggest a tissue-specific translational mechanism responsible for regulation of the expression of the PAF receptor mRNA in these tissues.     

Characterization of atrial natriuretic peptide in urine from rats treated with a neutral endopeptidase inhibitor.

To explore the mechanisms for the natriuretic effects of a neutral endopeptidase inhibitor, candoxatril, the concentration of atrial natriuretic peptide (ANP) and its molecular forms in the urine of Dahl salt-sensitive (S) rats were examined. Candoxatril-induced natriuresis (+120%, p less than 0.05) was associated with a marked increase in the urinary ANP excretion (+1200%, p less than 0.05). Analysis by Sephadex G-50 gel filtration revealed that molecular weight of the major fraction of immunoreactive (ir-) ANP in the plasma of candoxatril-treated Dahl S rats was 3K, whereas that in the urine was 2.5 K. Further analysis by reverse phase high performance liquid chromatography showed that ir-ANP in the plasma of Dahl S rats was alpha-rANP (1-28), while that in the urine from rats treated with candoxatril was alpha-rANP (1-25). These results indicate that candoxatril inhibits the complete degradation of ANP in the kidney, thereby increasing the amount of biologically active ANP reaching the distal nephron and contributing to natriuresis.     

Baculovirus p35 prevents developmentally programmed cell death and rescues a ced-9 mutant in the nematode Caenorhabditis elegans.

Programmed cell death, or apoptosis, occurs throughout the course of normal development in most animals and can also be elicited by a number of stimuli such as growth factor deprivation and viral infection. Certain morphological and biochemical characteristics of programmed cell death are similar among different tissues and species. During development of the nematode Caenorhabditis elegans, a single genetic pathway promotes the death of selected cells in a lineally fixed pattern. This pathway appears to be conserved among animal species. The baculovirus p35-encoding gene (p35) is an inhibitor of virus-induced apoptosis in insect cells. Here we demonstrate that expression of p35 in C. elegans prevents death of cells normally programmed to die. This suppression of developmentally programmed cell death results in appearance of extra surviving cells. Expression of p35 can rescue the embryonic lethality of a mutation in ced-9, an endogenous gene homologous to the mammalian apoptotic suppressor bcl-2, whose absence leads to ectopic cell deaths. These results support the hypothesis that viral infection can activate the same cell death pathway as is used during normal development and suggest that baculovirus p35 may act downstream or independently of ced-9 in this pathway.     

Retrotransposon-induced ectopic expression of the Om(2D) gene causes the eye-specific Om(M) phenotype in Drosophila ananassae

Optic morphology (Om) mutations in Drosophila ananassae map to at least 22 loci, which are scattered throughout the genome. Om mutations are all semidominant, neomorphic, nonpleiotropic, and associated with the insertion of a retrotransposon, tom. We have found that the Om(2D) gene encodes a novel protein containing histidine/proline repeats, and is ubiquitously expressed during embryogenesis. The Om(2D) RNA is not detected in wild-type eye imaginal discs, but is abundantly found in the center of the eye discs of Om(2D) mutants, where excessive cell death occurs. D. melanogaster flies transformed with the Om(2D) cDNA under control of the hsp70 promoter display abnormal eye morphology when heat-shocked at the third larval instar stage. These results suggest that the Om(2D) gene is not normally expressed in the eye imaginal discs, but its ectopic expression, induced by the tom element, in the eye disc of third instar larvae results in defects in adult eye morphology.