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D L Kirk

Publications and source records attributed to D L Kirk.

At least 37 records · Page 2Linked to original sources
PubMed1998

Enhancement of electrically evoked oto-acoustic emissions associated with low-frequency stimulus bias of the basilar membrane towards scala vestibuli.

Electrically evoked oto-acoustic emissions (EEOAEs) are sounds present in the ear canal when ac current is passed into the cochlea. EEOAEs are attributed to the activation of fast electromotile responses in outer hair cells (OHCs). An interesting property of EEOAEs is the phenomenon of "acoustic enhancement," where the emission amplitude is increased by moderate-level sound [D. C. Mountain and A. E. Hubbard, Hear. Res. 42, 195-202 (1989)]. In this report a form of enhancement is described which occurs with displacements of the basilar membrane toward scala vestibuli, during amplitude modulation of the EEOAE waveform by low-frequency tones. This "SV-bias enhancement" possibly consists of two components: (i) a low-level component induced by sound at levels which produce nonlinear growth of the cochlear microphonic and which may be equivalent to the "acoustic enhancement" described previously, and (ii) a high-level component which occurs at sound levels well above those which cause saturation of the cochlear microphonic. The low-level component could be explained by either an increased access of the extrinsically applied current to a membrane-based source of OHC motility, perhaps coupled with a reduction in negative feedback, or an increase in electromotile output during scala vestibuli displacements, but the origin of the high-level component is obscure.

Acoustic Stimulation↗
PubMed1997

Intron sequences provide a tool for high-resolution phylogenetic analysis of volvocine algae.

Three nuclear spliceosomal introns in conserved locations were amplified and sequenced from 28 strains representing 14 species and 4 genera of volvocalean green algae. Data derived from the three different introns yielded congruent results in nearly all cases. In pairwise comparisons, a spectrum of taxon-specific sequence differences ranging from complete identity to no significant similarity was observed, with the most distantly related organisms lacking any conserved elements apart from exon-intron boundaries and a pyrimidine-rich stretch near the 3' splice site. A metric (SI50), providing a measure of the degree of similarity of any pair of intron sequences, was defined and used to calculate phylogenetic distances between organisms whose introns displayed statistically significant similarities. The rate of sequences divergence in the introns was great enough to provide useful information about relationships among different geographical isolates of a single species, but in most cases was too great to provide reliable guides to relationships above the species level. A substitution rate of approximately 3 x 10(-8) per intron position per year was estimated, which is about 150-fold higher than in nuclear genes encoding rRNA and about 10-fold higher than the synonymous substitution rate in protein-coding regions. Thus, these homologous introns not only provide useful information about intraspecific phylogenetic relationships, but also illustrate the concept that different parts of a gene may be subject to extremely different intensities of selection. The intron data generated here (1) reliably resolve for the first time the relationships among the five most extensively studied strains of Volvox, (2) reveal that two other Volvox species may be more closely related than had previously been suspected, (3) confirm prior evidence that particular isolates of Eudorina elegans and Pleodorina illinoisensis appear to be sibling taxa, and (4) contribute to the resolution of several hitherto unsettled issues in Chlamydomonas taxonomy.

Amino Acid Sequence↗
PubMed1997

Microphonic and DPOAE measurements suggest a micromechanical mechanism for the 'bounce' phenomenon following low-frequency tones.

Neural auditory thresholds in the guinea pig can be temporarily improved by up to 6 dB about 2 min after the cessation of an moderately intense low-frequency tone (Kirk and Patuzzi, 1997). We have measured changes in the f2-f1 distortion product otoacoustic emission (DPOAE) and low-frequency microphonic potential in scala tympani before, during and after a low-frequency tone (200 Hz) to determine the cause of this so-called bounce phenomenon. In particular we have analysed the low-frequency microphonic waveform in detail to estimate changes in the maximal receptor current through the outer hair cells (OHCs), the sensitivity of the OHC forward transduction process and the change in OHC operating point on the mechano-electrical transduction transfer curve. Our results indicate that a 200 Hz tone changes the maximal current and sensitivity of the OHCs minimally, but more importantly, it transiently changes the operating point on the OHC transfer curve. In particular, the operating point changes are consistent with a movement of the OHC stereocilia away from the OHC basal body at the peak of the bounce. These changes detected using the microphonic potential are associated with changes in the level of the f2-f1 DPOAE that correlate well with the electrical measurements. We suggest that the shift in operating point is largely responsible for the increase in cochlear sensitivity, and is due to a disruption of the salt balance within the cochlea during the intense low-frequency tone.

Acoustic Stimulation↗
PubMed1997

Transient changes in cochlear potentials and DPOAEs after low-frequency tones: the 'two-minute bounce' revisited.

After exposure to a loud, non-traumatic low-frequency tone, auditory thresholds are elevated. Thresholds recover to normal in a non-monotonic manner, decreasing rapidly at first before increasing again, until they finally decrease monotonically towards normal. Although the transient elevation of thresholds after the initial improvement was originally called a 'bounce' by Hirsh and Ward (1952), Kemp (1986) suggests that the initial rapid recovery is the oddity: under some conditions a low-frequency tone can produce hypersensitivity in otoacoustic emissions, psychophysical thresholds, and perceived loudness (Kemp's 'bounce') without a later elevation of threshold (Hirsh and Ward's 'bounce'). Kemp also suggested that the transient hypersensitivity was caused by changes in the sensitivity of the active process within the cochlea. We have investigated the origin of this transient hypersensitivity (Kemp's bounce) in guinea pigs, recording cochlear potentials (CM, CAP, SP and EP) and otoacoustic emissions (DPOAEs at f2-f1, 2f1-f2, 2f2-2f1 and 3f1-2f2). Our results indicate that the bounce does not require neural activity, but is probably produced by non-neural cochlear mechanisms, possibly a transient decrease in the permeability of the organ of Corti which produces a small but significant change in standing current through outer hair cells. At least part of these changes, which are reduced as the stimulation frequency increases, and absent above 2 kHz, seem due to a small and transient movement of the cochlear partition towards scala tympani, probably due to a transient osmotic imbalance.

Acoustic Stimulation↗
PubMed1997

The genetic program for germ-soma differentiation in Volvox.

Volvox carteri possesses only two cell types: mortal somatic cells and potentially immortal asexual reproductive cells called gonidia. Mutational analysis indicates that three categories of genes play central roles in programming this germ-soma division of labor: First the gls genes function during embryogenesis to cause asymmetric divisions that produce large and small cells. Then the lag genes act in the large cells (gonidial initials) to repress functions required for somatic development while the regA locus acts in the small cells (somatic initials) to repress functions required for reproductive development. Transposon tagging and DNA transformation have recently been used to recover and characterize the glsA and regA genes, and the sequences of these genes lead to testable hypotheses about how they play their roles in germ-soma differentiation.

Algal Proteins↗
PubMed1996

Design of an FECG scalp electrode fetal heart rate monitor.

The design of a fetal heart rate (FHR) monitor using fetal electrocardiogram (FECG) scalp electrodes is described. It is shown that the design approach followed two stages: generation of FHR pulses at R-R intervals and FHR computation. The former uses a simple hardware approach for QRS detection and R-wave enhancement, while the latter requires a software implementation in order to produce FHR traces on a beat to beat basis. The QRS detection is based on bandpass filtering using switched mode capacitor technique; the R-wave enhancement and amplitude information are achieved by differentiation followed by fullwave rectification and peak detection. An adaptive threshold together with a comparator circuit are used to generate FHR pulses at R-R intervals. Beat to beat variations of FHR traces are produced by hardware and software implementation on a Z80 microprocessor board. Results obtained by the FHR monitor are evaluated and contrasted to other commercial FHR monitors.

Biomedical Engineering↗
PubMed1996

Frequency tuning and acoustic enhancement of electrically evoked otoacoustic emissions in the guinea pig cochlea.

Electrically evoked otoacoustic emissions (EEOAEs) were generated by ac stimulation in scala media of turns 1, 2, and 3 in the guinea pig cochlea. In each turn EEOAEs were recorded at frequencies up to and slightly above the estimated characteristic frequency (CF) of the stimulation site. Acoustic enhancement of EEOAEs was present at all emission frequencies in turns 2 and 3 but could be demonstrated in turn 1 only at emission frequencies that fell within a notch in the EEOAE tuning function. There was no evidence, in any turn, of a transition from enhancement to suppression as the emission frequency approached the CF of the stimulation site. The results were not consistent with the hypothesis [D. C. Mountain and A. E. Hubbard, Hear, Res. 42, 195-202 (1989)] that acoustic enhancement results from a reduction in the effectiveness of forward transduction in a negative feedback loop.

Acoustic Stimulation↗
PubMed1994

Nuclear transformation of Volvox carteri.

Stable nuclear transformation of Volvox carteri was achieved using the cloned V. carteri nitA+ gene (which encodes nitrate reductase) to complement a nitA- mutation. Following bombardment of mutant cells with plasmid-coated gold particles, putative transformants able to utilize nitrate as a nitrogen source were recovered with an efficiency of approximately 2.5 x 10(5). DNA analysis indicated that the plasmid integrated into the genome, often in multiple copies, at sites other than the nitA locus. Cotransformants were recovered with a frequency of 40-80% when cells were cobombarded with a selected and an unselected marker. Thus, V. carteri becomes one of the simplest multicellular organisms that is accessible to detailed molecular studies of genes regulating cellular differentiation and morphogenesis.

Blotting, Southern↗
PubMed1994

Germ cell specification in Volvox carteri.

Volvox carteri illustrates with diagrammatic clarity Weismann's concept of an immortal germline that produces a mortal soma that will carry it for a time, but then perish. Each V. carteri adult consists of about 16 asexual reproductive cells (gonidia) in the interior of a sphere that consists at its surface of about 2000 biflagellate somatic cells. When mature, each gonidium divides to form a juvenile with this same cellular composition. Half-way through their maturation, juveniles hatch out of the parenteral spheroid, whereupon parental somatic cells undergo programmed death while juvenile gonidia prepare for a new round of reproduction. The first visible step in V. carteri germ-soma differentiation is asymmetric cleavage, which sets apart large gonidial initials from small somatic initials. Experimental analysis indicates that it is a difference in size, not any difference in cytoplasmic quality, that determines whether a cell will become germinal or somatic. Mutational and molecular studies lead to the following model for the genetic control of the germ-soma dichotomy: first, the gls locus acts to cause asymmetric division; then large cells activate a set of lag loci that suppress expression of somatic genes, while small cells activate the regA locus that suppresses gonidial genes.

Animals↗
PubMed1994

Evidence for electrically evoked travelling waves in the guinea pig cochlea.

Electrically evoked compound action potentials (EECAP) were produced by the injection of pulsed sinusoidal current through metal electrodes into the basal turn. Plots of current threshold against frequency closely resembled conventional compound action potentials (CAP) audiograms for frequencies represented apically of the electrode location. EECAPs were masked by sound and CAPs were masked by current in a manner consistent with the generation of a propagated travelling wave.

Acoustic Stimulation↗
PubMed1993

Modulation of f2-f1: evidence for a GABA-ergic efferent system in apical cochlea of the guinea pig.

f2-f1, but not 2f1-f2, was reduced in amplitude during continuous stimulation of the test ear with the primary tones, and with single tones near the primary frequencies. Stimulation of the contralateral ear, either with broad band noise or with single tones near the primary frequencies, also reduced f2-f1. Ipsilateral and contralateral effects were additive and were restricted to the frequency range between about 2 kHz and 7 kHz. Contralateral, but not ipsilateral suppression, was blocked after systemic administration of strychnine. Ipsilateral suppression was eliminated by perfusion of the cochlea with tetrodotoxin. Both contralateral and ipsilateral suppression were abolished after perfusion of the cochlea with bicuculline. The results are evidence for a role for a GABA-ergic efferent system in the modulation of outer hair cell mechanics in the apical cochlea.

Acoustic Stimulation↗
PubMed1993

The relationship between cell size and cell fate in Volvox carteri.

In Volvox carteri development, visibly asymmetric cleavage divisions set apart large embryonic cells that will become asexual reproductive cells (gonidia) from smaller cells that will produce terminally differentiated somatic cells. Three mechanisms have been proposed to explain how asymmetric division leads to cell specification in Volvox: (a) by a direct effect of cell size (or a property derived from it) on cell specification, (b) by segregation of a cytoplasmic factor resembling germ plasm into large cells, and (c) by a combined effect of differences in cytoplasmic quality and cytoplasmic quantity. In this study a variety of V. carteri embryos with genetically and experimentally altered patterns of development were examined in an attempt to distinguish among these hypotheses. No evidence was found for regionally specialized cytoplasm that is essential for gonidial specification. In all cases studied, cells with a diameter > approximately 8 microns at the end of cleavage--no matter where or how these cells had been produced in the embryo--developed as gonidia. Instructive observations in this regard were obtained by three different experimental interventions. (a) When heat shock was used to interrupt cleavage prematurely, so that presumptive somatic cells were left much larger than they normally would be at the end of cleavage, most cells differentiated as gonidia. This result was obtained both with wild-type embryos that had already divided asymmetrically (and should have segregated any cytoplasmic determinants involved in cell specification) and with embryos of a mutant that normally produces only somatic cells. (b) When individual wild-type blastomeres were isolated at the 16-cell stage, both the anterior blastomeres that normally produce two gonidia each and the posterior blastomeres that normally produce no gonidia underwent modified cleavage patterns and each produced an average of one large cell that developed as a gonidium. (c) When large cells were created microsurgically in a region of the embryo that normally makes only somatic cells, these large cells became gonidia. These data argue strongly for a central role of cell size in germ/soma specification in Volvox carteri, but leave open the question of how differences in cell size are actually transduced into differences in gene expression.

Animals↗
PubMed1993

Jordan, an active Volvox transposable element similar to higher plant transposons.

We have isolated a 1595-bp transposable element from the multicellular green alga Volvox carteri following its insertion into the nitrate reductase (nitA) locus. This element, which we have named Jordan, has short (12-bp) terminal inverted repeats and creates a 3-bp target site duplication, like some higher plant transposons of the classic type. Contained within the first 200 bp of one end of the element are 55-bp inverted repeats, one of which begins with the terminal inverted repeat. Revertants of the transposon insertion into the nitA locus were obtained at a rate of approximately 10(-4) per Volvox embryo per generation. In each revertant examined, all transposon sequences were completely excised, but footprints containing both sets of duplicated bases, in addition to three to nine extra bases, were left behind. Jordan contains no significant open reading frames and so appears to be nonautonomous. DNA gel blot analysis indicates that Jordan is a member of a large family of homologous elements in the Volvox genome. We have isolated and characterized several of these homologs and found that they contain terminal very similar to those of Jordan. Efforts to utilize Jordan and its homologs as tools to tag and clone developmentally interesting genes of Volvox are discussed.

Base Sequence↗
PubMed1992

The nitrate reductase-encoding gene of Volvox carteri: map location, sequence and induction kinetics.

The nitrate reductase (NR) structural gene (nitA) of Volvox carteri has been cloned and characterized. There is a single copy of this gene in the genome, and RFLP (restriction-fragment length polymorphism) analysis assigns it to the previously defined nitA/chlR locus on linkage group IX, 20-30 cM from the two beta-tubulin-encoding loci. Determination of the 5871-nt sequence of the coding region of genomic clones, and comparisons to a cDNA sequence, revealed ten introns and eleven exons that encode a 864-aa polypeptide. Detailed comparisons with higher-plant and fungal NRs indicate that, whereas the aa sequence is strongly conserved within functional domains for the flavin adenine dinucleotide-, heme- and molybdenum-pterin cofactor-binding sites, substantial differences in the aa sequence occur in the N-terminal end and the two inter-domain regions. Two potential transcription start points 439 and 452 nt upstream from the start codon and a polyadenylation signal 355 nt downstream from the stop codon have been identified by primer-extension analysis and cDNA sequencing, respectively. Accumulation of the nitA transcript is both induced by nitrate and repressed by ammonium and urea: after the organism is transferred from ammonium to nitrate as the nitrogen source, a 3.6-kb NR transcript is readily detectable on Northern blots by 10 min, reaches maximum abundance by 30 min, and then rapidly declines to an intermediate level that is subsequently maintained. Substantial induction by nitrate is observed at the end of the dark portion of the daily light/dark cycle, but the inductive response peaks in the first hour of the light period.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1992

Molecular phylogeny of the volvocine flagellates.

Phylogenetic studies of approximately 2,000 bases of sequence from the large and small nuclear-encoded ribosomal RNAs are used to investigate the origins of the genus Volvox. The colonial and multicellular genera currently placed in the family Volvocaceae form a monophyletic group that is significantly closer phylogenetically to Chlamydomonas reinhardtii than it is to the other unicellular green flagellates that were tested, including Chlamydomonas eugametos, Chlorella pyrenoidosa, and Haematococcus lacustris. Statistical analysis of 251 phylogenetically informative nucleotide positions rejects the "volvocine lineage" hypothesis, which postulates a monophyletic evolutionary progression from unicellular organisms (such as Chlamydomonas), through colonial organisms (e.g., Gonium, Pandorina, Eudorina, and Pleodorina) demonstrating increasing size, cell number, and tendency toward cellular differentiation, to multicellular organisms having fully differentiated somatic and reproductive cells (in the genus Volvox). The genus Volvox appears not to be monophyletic. Volvox capensis falls outside a lineage containing other representatives of Volvox (V. aureus, V. carteri, and V. obversus), and both of these Volvox lineages are more closely related to certain colonial genera than they are to each other. This implies either a diphyletic origin of Volvox from different colonial volvocacean ancestors, a phylogenetic derivation of some of the colonial genera from a multicellular (i.e., Volvox) ancestor, or both. Considered together with previously published observations, these results suggest that the different levels of organizational and developmental complexity found in the Volvocaceae represent alternative stable states, among which evolutionary transitions have occurred several times during the phylogenetic history of this group.

Animals↗
PubMed1991

Identification of cell-type-specific genes of Volvox carteri and characterization of their expression during the asexual life cycle.

Volvox carteri possesses two morphologically and functionally distinct cell types: somatic cells and gonidia (asexual reproductive cells). To define the developmental programs involved in the differentiation of these two cell types, we have isolated 31 nonhomologous cDNA clones that hybridized to RNAs that were significantly more abundant in one cell type than the other. Details of the cell-type- and stage-specificity of expression of the transcripts detected by these cDNAs (plus five genes previously characterized by others) were examined by Northern-blot analysis. Accumulation patterns for the 19 gonidial transcripts fell into two distinct classes: transcripts of one gene were maximally abundant in very early cleavage, whereas transcripts of the other 18 did not reach maximal abundance until quite late in gonidial development. Similarly, the 12 somatic-cell-specific transcripts fell into two categories: transcripts of 5 "early" somatic genes became abundant soon after the completion of embryogenesis, whereas transcripts of 7 "late" somatic genes were not detected until later developmental stages. Expression of 3 other genes (two involved in flagellar development and one that encodes an extracellular matrix component) was also found to be restricted largely to somatic cells. These studies indicate that phenotypic differences between somatic cells and gonidia can be at least partially explained by differential regulation of RNA accumulation, and that there appear to be multiple patterns of accumulation of cell-type-specific transcripts within each cell type.

Animals↗
PubMed1991

Early and late gene expression programs in developing somatic cells of Volvox carteri.

In situ hybridization was used to resolve details of the temporal and spatial aspects of expression of a number of cell-type-specific genes of Volvox carteri. In confirmation of earlier results obtained by Northern-blot analysis, this study revealed that accumulation of transcripts of somatic genes was largely restricted to the somatic-cell lineage. In extension of the previous study, two distinct gene expression programs during somatic-cell development were more fully defined: "early" somatic genes were expressed in the somatic-cell lineage even before visible differentiation began, whereas "late" somatic genes were not expressed until after somatic cells had been visibly differentiated for more than a day and had already reached the presenescent-adult stage of development. We postulate that products of the early somatic genes may be involved in generalized somatic functions that are required throughout somatic-cell development, whereas the late somatic gene products may be involved in more specialized processes that characterize mature, parental somatic cells. The most significant finding of the present study is that transcripts of early somatic genes are significantly more abundant in presumptive somatic cells than in presumptive gonidia during embryonic stages, while the two cell types are linked by a network of cytoplasmic bridges. The fact that the two cell lineages can establish and maintain different transcript populations despite the extensive cytoplasmic continuity that exists between them is somewhat surprising and must be taken into account in future attempts to elucidate the way in which dichotomous differentiation is initiated in the organism.

Animals↗