PubMed · 4097425
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M Mlodzki. 1970. [Malabsorption syndrome].. https://pubmed.ncbi.nlm.nih.gov/4097425/
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Potassium is accumulated in plant vacuoles against an inside-positive membrane potential. The mechanism facilitating energized K+ transport has remained obscure. However, electrogenic activity of the inorganic pyrophosphatase (H(+)-PPase) at the vacuolar membrane is dependent on cytoplasmic K+, raising the possibility that the enzyme translocates K+ into the vacuole. Membrane currents generated by the H(+)-PPase were measured (using a patch clamp technique) in intact vacuoles isolated from Beta vulgaris storage tissue. A significant orthophosphate-dependent outward current mediated by the enzyme in reverse mode is evoked only when potassium is present at the vacuolar face of the tonoplast, suggesting that potassium is a translocated ion. Furthermore, current-voltage analysis of the effects of extravacuolar potassium and pH on the reversal potential of the H(+)-PPase-generated current points to direct translocation of K+ and H+ by the enzyme. Thus the H(+)-PPase represents a distinct class of eukaryote translocase and could facilitate vacuolar K+ accumulation in vivo.
Glucose transport is an important step in the regulation of glucose homeostasis. Two types of transport systems are described: active transport accumulates glucose in specific cells, whereas facilitative transport equilibrates blood glucose and intracellular glucose inside all mammalian cells. At the present time, different levels of facilitative transport regulation are known. Facilitative transport is achieved by 5 different isoforms; each isoform has its own characteristics and is subjected to tissue-specific regulation. Alteration of glucose transporters expression may be involved in a physiopathological situation such as diabetes which is characterized by insulin resistance of peripheral tissues and impaired insulin secretion by beta pancreatic cells. Thus, Glut 2 expression is reduced in the beta cells of diabetic rats. The reduction of Glut 2 expression correlates with, and may contribute to the loss of glucose-stimulated insulin secretion. However, Glut 2 expression in liver remains unaltered. The insulin resistance of peripheral tissues may be explained in adipose tissue by a decrease in Glut 4 expression. In skeletal muscle, Glut 4 expression remains constant whatever the physiological or physiopathological situation.