Absence of Small Conductance K+Channel (SK) Activity in Apical Membranes of Thick Ascending Limb and Cortical Collecting Duct in ROMK (Bartter's) Knockout Mice

Inherited hypokalaemic alkalosis with low blood pressure can be divided into two groups-Gitelman's syndrome, featuring hypocalciuria, hypomagnesaemia and milder clinical manifestations, and Bartter's syndrome, featuring hypercalciuria and early presentation with severe volume depletion. Mutations in the renal Na-Cl cotransporter have been shown to cause Gitelman's syndrome. We demonstrate linkage of Bartter's syndrome to the renal Na-K-2Cl cotransporter gene NKCC2, and identify frameshift or non-conservative missense mutations for this gene that co-segregate with the disease. These findings demonstrate the molecular basis of Bartter's syndrome, provide the basis for molecular classification of patients with inherited hypokalaemic alkalosis, and suggest potential phenotypes in heterozygous carriers of NKCC2 mutations.

Mutations in the Na-K-2Cl cotransporter (NKCC2), a mediator of renal salt reabsorption, cause Bartter's syndrome, featuring salt wasting, hypokalaemic alkalosis, hypercalciuria and low blood pressure. NKCC2 mutations can be excluded in some Bartter's kindreds, prompting examination of regulators of cotransporter activity. One regulator is believed to be ROMK, an ATP-sensitive K+ channel that 'recycles' reabsorbed K+ back to the tubule lumen. Examination of the ROMK gene reveals mutations that co-segregate with the disease and disrupt ROMK function in four Bartter's kindreds. Our findings establish the genetic heterogeneity of Bartter's syndrome, and demonstrate the physiologic role of ROMK in vivo.

The past three years have seen remarkable progress in research on the molecular basis of inward rectification, with significant implications for basic understanding and pharmacological manipulation of cellular excitability. Expression cloning of the first inward rectifier K channel (Kir) genes provided the necessary break-through that has led to isolation of a family of related clones encoding channels with the essential functional properties of classical inward rectifiers, ATP-sensitive K channels, and muscarinic receptor-activated K channels. High-level expression of cloned channels led to the discovery that classical inward so-called anomalous rectification is caused by voltage-dependent block of the channel by polyamines and Mg2+ ions, and it is now clear that a similar mechanism results in inward rectification of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate receptor channels. Knowledge of the primary structures of Kir channels and the ability to mutate them also has led to the determination of many of the structural requirements of inward rectification.