Skeletal muscle and brain isoforms of a beta-subunit of human voltage-dependent calcium channels are encoded by a single gene

Calcium flux responses mediated by voltage-dependent calcium channels have been studied in transverse tubule membrane vesicles from rabbit skeletal muscle. Vesicles were loaded with 45Ca2+, and membrane potentials were generated by establishing potassium gradients across the membrane in the presence of valinomycin. After the membranes were polarized to an estimated −80 mV to approximate the resting state of the cell, a significant 45Ca2+ efflux occurred upon subsequent depolarization to −60 mV. The efflux response was modulated by activators and inhibitors of slow, dihydropyridine-sensitive calcium channels, being inhibited by inorganic calcium channel blockers, verapamil, nifedipine, and (−)-SDZ 202 – 791 and potentiated by the dihydropyridine agonists (±)-Bay K8644 and (+)-SDZ 202 – 791. These results demonstrate that calcium channels in transverse tubule membranes can open to mediate calcium flux in the same range of membrane potential as the late afterpotentials that occur during tetanic contractions of intact muscle fibres.Key words: Ca2+ channels, dihydropyridines, skeletal muscle (rabbit), 45Ca2+ flux, late afterpotentials.

Download Full-text

Molecular characterization of renal calcium channel beta-subunit transcripts

AJP Renal Physiology ◽

10.1152/ajprenal.1995.268.3.f525 ◽

1995 ◽

Vol 268 (3) ◽

pp. F525-F531 ◽

Cited By ~ 6

Author(s):

A. S. Yu ◽

M. Boim ◽

S. C. Hebert ◽

A. Castellano ◽

E. Perez-Reyes ◽

...

Keyword(s):

Calcium Channels ◽

Calcium Channel ◽

Rat Kidney ◽

Beta Subunit ◽

Kidney Cortex ◽

Calcium Excretion ◽

Gene Transcript ◽

Degenerate Primers ◽

Voltage Dependent ◽

Voltage Dependent Calcium Channels

An apical, hormone-regulated, calcium entry channel in the distal convoluted tubule and/or connecting tubule (DCT/CNT) is thought to play an important role in controlling renal calcium excretion. We previously identified a gene transcript encoding the pore-forming alpha 1-subunit of a calcium channel (alpha 1A, or CaCh4) which may be a candidate for such a molecule. The properties of voltage-dependent calcium channels are known to be modulated by their beta-subunits. To identify the accessory beta-subunit of DCT/CNT calcium channels, degenerate primers based on published beta-subunit sequences were used to amplify rat kidney cDNA by the polymerase chain reaction (PCR), and the products were subcloned and sequenced. Alternatively spliced transcripts of three beta-subunit genes (beta 2, beta 3, and beta 4) were identified. Northern blot analysis indicated that beta 4-subunit is preferentially expressed in kidney cortex. Transcripts of all three beta-subunit genes were detected by PCR in microdissected nephron segments, but only beta 4-subunit was found in DCT/CNT. As the beta 4- and alpha 1A-subunits colocalize to the DCT/CNT, we hypothesize that they may be constituent subunits of a renal calcium channel regulated by a hormone(s).

Download Full-text