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S Taffet

Publications and source records attributed to S Taffet.

6 recordsLinked to original sources
PubMed2001

Rectification of the background potassium current: a determinant of rotor dynamics in ventricular fibrillation.

Ventricular fibrillation (VF) is the leading cause of sudden cardiac death. Yet, the mechanisms of VF remain elusive. Pixel-by-pixel spectral analysis of optical signals was carried out in video imaging experiments using a potentiometric dye in the Langendorff-perfused guinea pig heart. Dominant frequencies (peak with maximal power) were distributed throughout the ventricles in clearly demarcated domains. The fastest domain (25 to 32 Hz) was always on the anterior left ventricular (LV) wall and was shown to result from persistent rotor activity. Intermittent block and breakage of wavefronts at specific locations in the periphery of such rotors were responsible for the domain organization. Patch-clamping of ventricular myocytes from the LV and the right ventricle (RV) demonstrated an LV-to-RV drop in the amplitude of the outward component of the background rectifier current (I(B)). Computer simulations suggested that rotor stability in LV resulted from relatively small rectification of I(B) (presumably I(K1)), whereas instability, termination, and wavebreaks in RV were a consequence of strong rectification. This study provides new evidence in the isolated guinea pig heart that a persistent high-frequency rotor in the LV maintains VF, and that spatially distributed gradients in I(K1) density represent a robust ionic mechanism for rotor stabilization and wavefront fragmentation.

Animals↗
PubMed1993

A structural basis for the unequal sensitivity of the major cardiac and liver gap junctions to intracellular acidification: the carboxyl tail length.

The regulation of junctional conductance (Gi) of the major cardiac (connexin43; Cx43) and liver (connexin32; Cx32) gap junction proteins by intracellular hydrogen ion concentration (pH; pHi), as well as well as that of a truncation mutant of Cx43 (M257) with 125 amino acids deleted from the COOH terminus, was characterized in pairs of Xenopus laevis oocytes expressing homologous channels. Oocytes were injected with 40 nl mRNAs (2 micrograms/microliters) encoding the respective proteins; subsequently, cells were stripped, paired, and incubated for 20-24 h. Gj was measured in oocyte pairs using the dual electrode voltage-clamp technique, while pHi was recorded simultaneously in the unstimulated cell by means of a proton-selective microelectrode. Because initial experiments showed that the pH-sensitive microelectrode responded more appropriately to acetate than to CO2 acidification, oocytes expressing Cx32 and wild type and mutant Cx43 were exposed to a sodium acetate saline, which was balanced to various levels of pH using NaOH and HCl. pH was changed in a stepwise manner, and quasi-steady-state Gj -pHi relationships were constructed from data collected at each step after both Gj and pHi had reached their respective asymptotic values. A moderate but significant increase of Gj was observed in Cx43 pairs as pHi decreased from 7.2 to 6.8. In both Cx32 and M257 pairs, Gj increased significantly over a wider pH range (i.e., between 7.2 and 6.3). Further acidification reversibly reduced Gj to zero in all oocyte pairs. Pooled data for the individual connexins obtained during uncoupling were fitted by the Hill equation; apparent 50%-maximum (pK;pKa) values were 6.6 and 6.1 for Cx43 and Cx32, respectively, and Hill coefficients were 4.2 for Cx43 and 6.2 for Cx32. Like Cx32, M257 had a more acidic pKa (6.1) and steeper Hill coefficient (6.0) than wild type Cx43. The pKa and Hill coefficient of M257 were very similar to those of Cx32. These experiments provide the first direct comparison of the effects of acidification on Gj in oocyte pairs expressing Cx43 or Cx32. The results indicate that structural differences in the connexins are the basis for their unequal sensitivity to intracellular acidification in vivo. The data further suggest that a common pH gating mechanism may exist between amino acid residues 1 and 256 in both Cx32 and Cx43. However, the longer carboxyl tail of Cx43 relative to Cx32 or M257 provides additional means to facilitate acidification-induced gating; its presence shifts the pKa from 6.1 (Cx32 and M257) to 6.6 (Cx43) in the conductance of these channels.

Animals↗
PubMed1992

Immunolocalization and expression of functional and nonfunctional cell-to-cell channels from wild-type and mutant rat heart connexin43 cDNA.

The carboxyl terminal cytoplasmic domain of distinct gap junction proteins may play an important role in assembly of functional channels as well as differential responsiveness to pH, voltage, and intracellular second messengers. Oligonucleotide-directed site-specific mutagenesis in a paired Xenopus laevis oocyte expression system was used to examine the expression of mRNAs encoding wild-type and carboxyl terminal mutant connexin43 (Cx43) proteins. Oocytes were stripped, injected with mRNA or distilled water (dH2O), preincubated for 16-20 hours, and then paired for 5-10 hours; this process was followed by electrophysiological recording using the dual voltage-clamp technique. Initial experiments compared the relative junctional conductances (Gjs) in oocyte pairs expressing Cx43 (382 amino acid residues) and two truncated mutants lacking most or a portion of the cytoplasmic carboxyl terminal. The shortest mutant (M241) contained 240 amino acid residues and was devoid of all phosphorylatable serine residues in the cytoplasmic tail; its length approximated the length of liver connexin26. The longest mutant (M257) tested contained 256 amino acid residues, including two serine residues. Oocyte pairs expressing M241 yielded a Gj similar to that of oocytes injected with dH2O, whereas M257 yielded a Gj similar to that of oocytes injected with Cx43. Immunoprecipitation studies showed that Cx43, M257, and M241 proteins were readily detectable in oocytes injected with their respective mRNAs, indicating that the lack of Gj observed with the M241 mRNA was not due to reduced translation. Immunocytochemical studies revealed that wild-type and both truncated mutants were localized to the area of cell-to-cell contact between the paired oocytes, indicating that protein targeting to the membrane was not inhibited in oocytes injected with M241 mRNA. Oocyte pairs expressing mutants in which serine residues were replaced with nonphosphorylatable amino acids (serine codon No. 255 AGC was converted to GCC, alanine, designated as M255S----A, and serine codon No. 244 AGC was converted to GGC, glycine, designated as M244S----G) showed Gjs similar to M257, indicating that these serine residues and, by inference, their phosphorylation state are not critical for expression of functional channels. The importance of the length of the carboxyl terminus was assessed by comparing the Gjs in a series of mutants that were intermediate in length between M257 and M241. Gradual shortening of the carboxyl terminus produced a gradual reduction of Gj relative to M257. However, simple deletion of amino acid residues 241-257 from the wild-type Cx43 did not affect Gj relative to M257.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1977

The association of rubeosis iridis with endothelialisation of the anterior chamber: report of a clinical case with histopathological review of 16 additional cases.

A patient is reported in whom the clinical observation of extension of the corneal endothelium and Descemet's membrane in association with rubeosis iridis was confirmed by histopathological examination. In a study of eyes which had been removed and demonstrated rubeosis iridis and secondary glaucoma in 16 additional eyes revealed extension of the corneal endothelium and Descemet's membrane on to the anterior iris surface. In all instances the transition occurred at the pseudoangle formed by the attachment of peripheral anterior synechiae. The suggestion is made that the fibrovascular membrane of rubeosis iridis is the stimulus which excites the extension of the endothelium on to the iris surface. The association between rubeosis iridis and extension of the corneal endothelium and Descemet's membrane on to the iris has not been previously appreciated.

Adolescent↗