Polystyrene spherules averaging 0.5 millimeter in diameter (range 0.1 to 2 millimeters) are abundant in the coastal waters of southern New England. Two types are present, a crystalline (clear) form and a white, opaque form with pigmentation resulting from a diene rubber. The spherules have bacteria on their surfaces and contain polychlorinated biphenyls, apparently absorbed from ambient seawater, in a concentration of 5 parts per million. White, opaque spherules are selectively consumed by 8 species of fish out of 14 species examined, and a chaetognath. Ingestion of the plastic may lead to intestinal blockage in smaller fish.

Amiloride-sensitive ion channels are formed from homo-or heteromeric combinations of subunits from the epithelial Na ؉ channel (ENaC)/degenerin superfamily, which also includes the acid-sensitive ion channel (ASIC) family. These channel subunits share sequence homology and topology. In this study, we have demonstrated, using confocal fluorescence resonance energy transfer microscopy and co-immunoprecipitation, that ASIC and ENaC subunits are capable of forming cross-clade intermolecular interactions. We have also shown that combinations of ASIC1 with ENaC subunits exhibit novel electrophysiological characteristics compared with ASIC1 alone. The results of this study suggest that heteromeric complexes of ASIC and ENaC subunits may underlie the diversity of amiloride-sensitive cation conductances observed in a wide variety of tissues and cell types where co-expression of ASIC and ENaC subunits has been observed.Amiloride-sensitive Na ϩ channels are formed from combinations of subunits from the epithelial Na ϩ channel (ENaC) 2 / degenerin superfamily, which includes ENaC, degenerin, and acid-sensitive ion channels (ASICs). Members of this superfamily of more than 60 identified subunits share a common membrane topology, with relatively short intracellular N and C termini (ϳ100 amino acids), two transmembrane-spanning ␣-helices, and a large extracellular loop (ϳ400 amino acids) (1). ENaC/degenerin subunits share 15-20% sequence identity across the entire superfamily. Within individual subfamilies, the sequence identity rises (ϳ30% identity for ENaCs, 30% for degenerins, and 45-60% for ASICs) (2).Amiloride-sensitive ion channels have been identified in a wide variety of cell lines and tissue types. ENaCs were initially isolated from the kidney, where Na ϩ reabsorption in the distal collecting duct is required for water reabsorption and concentration of the urine (3). Inhibition of ENaC with amiloride therefore leads to diuresis. ENaCs have since been identified in several other tissues, including vascular smooth muscle, oocytes, lymphocytes, neurons, osteoblasts, pancreas, testis, ovary, heart, lung, and urinary bladder (4 -11). ASIC subunits were initially identified in the brain and dorsal root ganglion by sequence homology with known ENaC subunits (12). They have since been identified in the central and peripheral nervous system and in the cardiac and skeletal myocytes (13-17). Mounting evidence indicates that ASIC and ENaC subunits are co-expressed in multiple tissues and cell types, including pheochromocytoma cells, osteoblasts, chondocytes, astrocytes, retina, lung, kidney, taste receptors, and dorsal root ganglion cells (Table 1) (9,14,(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34).The electrophysiological characteristics of cells expressing amiloride-sensitive cation conductances vary widely, and regulation of ENaCs and ASICs occurs through several mechanisms. Pharmacological inhibitors of channel function include small molecules, such as amiloride, or peptide toxins, such as psalmoto...

Spt5p is a universally conserved transcription factor that plays multiple roles in eukaryotic transcription elongation. Spt5p forms a heterodimer with Spt4p and collaborates with other transcription factors to pause or promote RNA polymerase II transcription elongation. We have shown previously that Spt4p and Spt5p also influence synthesis of ribosomal RNA by RNA polymerase (Pol) I; however, previous studies only characterized defects in Pol I transcription induced by deletion of SPT4. Here we describe two new, partially active mutations in SPT5 and use these mutant strains to characterize the effect of Spt5p on Pol I transcription. Genetic interactions between spt5 and rpa49⌬ mutations together with measurements of ribosomal RNA synthesis rates, rDNA copy number, and Pol I occupancy of the rDNA demonstrate that Spt5p plays both positive and negative roles in transcription by Pol I. Electron microscopic analysis of mutant and WT strains confirms these observations and supports the model that Spt4/5 may contribute to pausing of RNA polymerase I early during transcription elongation but promotes transcription elongation downstream of the pause(s). These findings bolster the model that Spt5p and related homologues serve diverse critical roles in the control of transcription.

Epithelial Na؉ channels (ENaC) regulate salt and water re-absorption across the apical membrane of absorptive epithelia such as the kidney, colon, and lung. Structure-function studies have suggested that the second transmembrane domain (M2) and the adjacent pre-and post-M2 regions are involved in channel pore formation, cation selectivity, and amiloride sensitivity. Because Na ؉ selectivity, unitary Na ؉ conductance (␥ Na ), and amiloride sensitivity of ␦-ENaC are strikingly different from those of ␣-ENaC, the hypothesis that the pre-H2 domain may contribute to these characterizations has been examined by swapping the pre-H2, H2, and both (pre-H2؉H2) domains of ␦-and ␣-ENaCs. Whole-cell and single channel results showed that the permeation ratio of Li ؉ and Na ؉ (P Li /P Na ) for the swap ␣ chimeras coexpressed with ␤␥-ENaC in Xenopus oocytes decreased significantly. In contrast, the ratio of P Li /P Na for the swap ␦ constructs was not significantly altered. Single channel studies confirmed that swapping of the H2 and the pre-H2؉H2 domains increased the ␥ Na of ␣-ENaC but decreased the ␥ Na of ␦-ENaC. A significant increment in the apparent inhibitory dissociation constant for amiloride (K i amil ) was observed in the ␣ chimeras by swapping the pre-H2, H2, and pre-H2؉H2 domains. In contrast, a striking decline of K i amil was obtained in the chimeric ␦ constructs with substitution of the H2 and pre-H2؉H2 domains. Our results demonstrate that the pre-H2 domain, combined with the H2 domain, contributes to the P Li /P Na ratio, single channel Na ؉ conductance, and amiloride sensitivity of ␣-and ␦-ENaCs.The epithelial Na ϩ channel (ENaC) 1 was the first subgroup of the ENaC/DEG superfamily cloned from mammals. The topology of ENaC/DEG comprises two short N-and C-terminal intracellular tails, two hydrophobic membrane-spanning domains (M1 and M2), and a large, extracellular loop with two (or three) cysteine-rich domains (1, 2). There is an overall ϳ37% amino acid identity between ␣-, ␦-, ␤-, and ␥-subunits. Both the ␣-and ␦-subunits can form independent conducting channels with similar amiloride sensitivities and ion selectivities to those co-expressed with ␤␥-subunits. The ␤-and ␥-subunits are modifying subunits that regulate the trafficking and conductance of ␣-and ␦-ENaCs (1, 2). The ␣␤␥-ENaC has a higher permeability to Na ϩ and Li ϩ compared with the other alkali metals. For example, ␣␤␥-ENaC has a permeability ratio of P Li /P Na up to 2, but the channel is virtually impermeant to K ϩ ions (1, 2). Similar to other ENaC/DEG superfamily members, ENaC is very sensitive to amiloride, displaying an apparent inhibitory dissociation constant (K i amil ) in the nanomolar range (1, 2).Previous publications (1, 2) have demonstrated that the pre-M2 region of ␣-ENaC, more precisely, the second hydrophobic domain (H2) preceding the M2 region, serves as the outer mouth of the ENaC pore and is involved in channel gating. An amiloride-binding site has also been identified functionally in the H2 domain in addition to the one mo...

Low calcium (Ca) contents and low calcium:phosphorus (Ca:P) ratios of mealworm larvae and house crickets can result in imbalances of Ca and phosphorus (P) in diets of avian species when these insects form more than a minor proportion of the diet. Appropriate dietary Ca and Ca:P levels are particularly important for normal growth and bone development in chicks, especially of long‐legged species such as bustards. Two experiments were carried out to evaluate the efficacy of a selection of practicable dietary options for increasing the Ca levels and Ca:P ratios of cultured mealworm larvae and immature house crickets used for feeding bustards. Dietary treatments contained higher levels of Ca than the insects' standard culture diet components but similar P levels. Dietary treatment significantly increased Ca level and Ca:P ratio of both mealworm larvae and immature house crickets but did not affect P content of either species. Acceptable insect Ca and Ca:P levels were achieved by maintaining insects on commercial high‐Ca diet products for as little as 24 hours. Other factors that may have influenced the Ca levels of mealworm larvae and house crickets include physical form and overall nutrient composition of their diets. Zoo Biol 19:1–9, 2000. © 2000 Wiley‐Liss, Inc.