Involvement of Ca2+ buffering and Na+/Ca2+ exchange in the positive staircase of contraction in guinea-pig ventricular myocytes.

The mean sarcomere length (SL) of guinea-pig cardiac myocytes was recorded simultaneously with the whole-cell current under voltage-clamp conditions. After blocking both sarcoplasmic reticulum (SR) and L-type Ca(2+) channels with ryanodine, cyclopiazonic acid and nicardipine, strong depolarizing pulses induced only the tonic component of SL shortening through the reverse mode of Na(+)/Ca(2+) exchange (NCX). A positive staircase of SL shortening was observed on applying a train pulses to +60~+100 mV at 2 Hz and trans-membrane Ca(2+) flux was calculated from the time integral of the Na(+)/Ca(2+) exchange current ( I(NCX)). Changes in cytosolic [Ca(2+)] ([Ca(2+)](i)) were determined indirectly using the experimental [Ca(2+)](i)/SL relationship. Cellular Ca(2+) buffering was characterized by a lumped single-component system with a maximum binding capacity of 200 micro M and a dissociation constant of 613 nM. Despite the decrease in driving force, the amplitude of the outwards I(NCX) at +60 mV gradually increased along with the positive staircase. The model simulation suggested that this increase of outwards I(NCX) is caused by a dramatic increase in Ca(2+)-mediated activation of NCX.

Sequence-based structural features between Kvlqt1 and Tapa1 on mouse chromosome 7F4/F5 corresponding to the Beckwith-Wiedemann syndrome region on human 11p15.5: long-stretches of unusually well conserved intronic sequences of kvlqt1 between mouse and human.

Mouse chromosome 7F4/F5 is a syntenic locus of human 11p15.5 in which many imprinted genes are clustered. Transmission of aberrant human 11p15.5 or duplicated 11p causes Beckwith-Wiedemann syndrome (BWS) depending on which parent the chromosome is derived from. To analyze a syntenic mouse locus corresponding to human 11p15.5, mouse BAC contigs were constructed between Nap2 and Tapa1, in which 390 kb was sequenced between Kvlqt1 and Tapa1. An unexpected finding was that of highly conserved intronic sequences of Kvlqt1 between mouse and human, and their homologies came up to at least 160 kb because the length of this gene extended to 350 kb, suggesting the possibility of some functional constraint due to transcriptional and/or post-transcriptional regulation of this region. Many expressed sequence tags (ESTs) were mapped on this locus. Three genes, Lit1 (Kvlqt1-AS), Mtr1 and Tssc4, were identified and characterized. Lit1 is an antisense-transcript of Kvlqt1 and paternally expressed and maternally methylated throughout the developmental stage. The position where Lit1 exists corresponded to a highly conserved region between mouse and human. This transcript extends at least 60 kb from downstream to upstream of exon 10 in Kvlqt1. Tssc4 and Mtr1 carried putative open reading frames but neither was imprinted. Further characterization of this locus based on the sequence comparison between mouse and human will contribute valuable information towards resolving the mechanism of the occurrence of BWS and the associated childhood tumor.