No abstract

Everted gut-sacs prepared from segments of the proximal small intestine of young rabbits, rats and guinea pigs transport Ca45 in vitro from the mucosal to the serosal surfaces against concentration gradients. The active transport mechanism is limited in capacity, is dependent on oxidative phosphorylation, and appears to be relatively specific for Ca++ and Mg++ in contrast to Sr++ and Ba++. Vitamin D deprivation in rabbits and rats markedly impairs the capacity for active Ca45 transport in vitro. The vitamin thus has an effect directly on the upper small intestine. Neither the active Ca45 transport nor the effect of vitamin D on the transport can be explained by an accumulation of citrate and the formation of the calcium-citrate complex.

The purpose of this investigation was to correlate the viscoelastic properties and lipid fluidity of the red blood cell membrane to its lipid composition. The viscoelastic properties of human red cells that had been enriched or depleted in cholesterol were determined by the micropipette technique. The lipid fluidity of the outer and inner leaflets of the erythrocyte membrane was concurrently assessed by steady state fluorescence depolarization. The elastic modulus and the viscosity moduli of the erythrocyte membrane showed no significant differences between the cholesterol-modified and the control cells. Cholesterol enrichment decreased the lipid fluidity of the outer membrane leaflet alone, and cholesterol depletion increased the fluidity mainly of the inner leaflet.

LIPID-PROTEIN INTERACTIONS INMEMBRANES This review considers the basic biological problem of how the lipids and proteins of hepatocyte plasma membranes interact to mediate and regulate membrane functions. The following general hypothesis, which is applicable to all natural membranes, is a suitable framework for the discussion, inasmuch as it rests on a considerable body of experimental evidence. MEMBRANE MEMBRANE PROTEINS influence LIPIDS(and the specific -(and their functions properties) mediated by them)The influence of endogenous membrane enzymes, such as phospholipases (1-4), methyltransferases (5, 6), and fatty acid desaturases (7-lo), on membrane phospholipids is well-documented. Such reactions can alter the composition and properties of the bilayer lipid environment and thereby affect the functions of those membrane proteins ("integral" proteins) which are embedded in or traverse the lipid core of the membrane. A key property of the lipids, in this regard, is the "fluidity", or general motional freedom of the lipid molecules. A growing body of evidence indicates that the fluidity of the lipid environment influences the activity of such physiologically important membrane enzymes as the (Na+ + Ka+)-dependent adenosine triphosphatase (11-16) and hormoneresponsive adenylate cyclase (16-20). The fluidity also modulates transmembrane transport processes (21-24) and is a determinant of the passive permeability characteristic of a given bilayer (23, 25-28).The interactions of membrane proteins and lipids implicit in the general hypothesis above could provide regulatory loops for the control of important cell functions. For example, recent evidence indicates that calcium ion