Effects of potassium-free media and ouabain on epithelial cell composition in toad urinary bladder studied with X-ray microanalysis

Summary The technique of X-ray microanalysis was used to study the composition of toad urinary bladder epithelial cells incubated in Na Ringer's and K-free medium, with and without ouabain. Following incubation under short-circuit conditions, portions of tissue were coated with an external albumin standard and plunge-frozen. Cryosections were freeze-dried and analyzed. In Na Ringer's, granular and basal cells, and also the basal portion of the goblet cells, had similar water and ion compositions. In contrast, mitochondria-rich cells contained less Cl and Na. On average, the granular cells and a subpopulation of the basal cells lost K and gained Na after ouabain and in K-free medium alone. However, there was considerable variation from cell to cell in the responses, indicating differences between cells in the availabilities of ion pathways, either as a consequence of differences in the numbers of such pathways or in their control. In contrast, the compositions of both the low Cl, mitochondria-rich cells and a sub-population of the basal cells were little affected by the different incubation conditions. This is consistent with a comparatively low Na permeability of these cells. The results also indicate that (i) much, if not all, of the K in the dominant cell type, the granular cells, is potentially exchangeable with serosal medium Na, and (ii) Na is accumulated from the serosal medium under K-free conditions. They also provide information about the role of the (Na–K)-ATPase in the maintenance of cellular K in K-free medium, being consistent with other evidence that removal of serosal medium K inhibits transepithelial Na transport by decreasing Na entry to the cells from the mucosal medium, rather than solely by inhibiting the basolateral membrane (Na–K)-ATPase.     

Contributions of secondary active transport processes to membrane potentials

Summary Equations are developed to examine the effects of secondary active transport processes on the steady-state membrane potential of symmetrical cells. It is shown that, with suitable modifications, equations of the type developed by Goldman, Hodgkin and Katz may be derived to accommodate the contributions to the membrane potential of both electroneutral and electrogenic transporters. Where the membrane potential is function of the dominant medium ions (Na, K, and Cl), other contributions can come only from an electrogenic Na pump and from neutral co- and counter-transporters if, and only if, these involve the dominant ions. Experimental approaches to measure the parameters necessary to solve the equations developed here are discussed.     

Effects of vasopressin on the water and ionic composition of toad bladder epithelial cells

Summary Isolated sheets of epithelial cells as well as epithelial cells scraped from paired hemibladders mounted in chambers both showed significant increases in water, sodium and chloride contents after exposure to vasopressin (100 mU/ml), without any change in potassium content. In the isolated cells these changes were prevented by amiloride (10^–5 m), suggesting that the gain of sodium after vasopressin occurs across the mucosal membrane. This hypothesis was confirmed in experiments in which it was found that, in hemibladders mounted in chambers and bathed on their mucosal surface by sodium Ringer's with²⁴Na, the gains of chemical sodium and²⁴Na after vasopressin were equivalent.     

FRIGIDA and related proteins have a conserved central domain and family specific N- and C- terminal regions that are functionally important

Arabidopsis accessions are either winter-annuals, which require cold winter temperatures for spring flowering, or rapid-cycling summer annuals. Typically, winter annual accessions have functional FRIGIDA (FRI) and FRIGIDA-LIKE 1 (FRL1) proteins that promote high expression of FLOWERING LOCUS C (FLC), which prevents flowering until after winter. In contrast, many rapid-cycling accessions have low FLC levels because FRI is inactive. Using biochemical, functional and bioinformatic approaches, we show that FRI and FRL1 contain a stable, central domain that is conserved across the FRI superfamily. This core domain is monomeric in solution and primarily α-helical. We analysed the ability of several FRI deletion constructs to function in Arabidopsis plants. Our findings suggest that the C-terminus, which is predicted to be disordered, is required for FRI to promote FLC expression and may mediate protein:protein interactions. The contribution of the FRI N-terminus appears to be limited, as constructs missing these residues retained significant activity when expressed at high levels. The important N- and C-terminal regions differ between members of the FRI superfamily and sequence analysis identified five FRI families with distinct expression patterns in Arabidopsis, suggesting the families have separate biological roles.