Luers Claus, Fialka Florian, Elgner Andreas, Zhu Danan, Kockskämper Jens, von Lewinski Dirk, Pieske Burkert
Department of Cardiology and Pneumology, University of Goettingen, Robert-Koch-Strasse 40, 37075 Goettingen, Germany.
Cardiovasc Res. 2005 Dec 1;68(3):454-63. doi: 10.1016/j.cardiores.2005.07.001. Epub 2005 Aug 15.
Rabbit ventricular myocardium is characterized by a biphasic response to stretch with an initial, rapid increase in force followed by a delayed, slow increase in force (slow force response, SFR). The initial phase is attributed to increased myofilament Ca(2+) sensitivity, but the mechanisms of the delayed phase are only incompletely understood. We tested whether stretch-dependent stimulation of Na(+)/H(+) exchange (NHE1) and consecutive changes in pH(i) and/or Na(+) may underlie the SFR.
Isometric contractions of rabbit ventricular muscles were recorded in bicarbonate-containing Tyrode's (Tyrode) or bicarbonate-free HEPES-buffered solution (HEPES). Muscles were loaded with the Ca(2+) indicator aequorin, the pH indicator BCECF, or the Na(+) indicator SBFI and rapidly stretched from 88% (L(88)) to 98% (L(98)) of optimal length. The resulting immediate and slow increases in twitch force (1st phase and SFR) as well as changes in Ca(2+), Na(+), or pH(i) were quantified before and after inhibition of NHE1 by HOE 642 (3 microM) or reverse-mode Na(+)/Ca(2+) exchange (NCX) by KB-R 7943 (5 microM).
In both Tyrode (n=21) and HEPES (n=22), developed force increased to approximately 160% during the 1st phase followed by a further increase to approximately 205% during the SFR. The SFR was accompanied by a 21% increase of the aequorin light transient (n=4; normalized to the 1st phase) and a approximately 3 mM increase in Na(+) (n=4-7). The SFR was also associated with an increase in pH(i). However, this increase was delayed and was significant only after the SFR had reached its maximum. The delayed pH(i) increase was larger in HEPES than in Tyrode. HOE 642 and/or KB-R 7943 reduced the SFR by approximately 30-40%. In addition, HOE 642 diminished the stretch-mediated elevation of Na(+) by 72% and the delayed alkalinization.
The data are consistent with the hypothesis that SFR results from increases in Ca(2+) secondary to altered flux via NCX in part resulting from increases in Na(+) mediated by NHE1.
兔心室肌对拉伸具有双相反应，最初是力迅速快速增加，随后是力延迟缓慢增加（慢力反应，SFR）。初始阶段归因于肌丝Ca(2+)敏感性增加，但延迟阶段的机制仅得到不完全理解。我们测试了拉伸依赖性刺激Na(+)/H(+)交换（NHE1）以及随后细胞内pH值（pH(i)）和/或细胞内[Na(+)]（Na(+)）的变化是否可能是SFR的基础。
在含碳酸氢盐的台氏液（Tyrode液）或无碳酸氢盐的HEPES缓冲溶液（HEPES）中记录兔心室肌的等长收缩。肌肉加载Ca(2+)指示剂水母发光蛋白、pH指示剂BCECF或Na(+)指示剂SBFI，并从最佳长度的88%（L(88)）迅速拉伸至98%（L(98)）。在HOE 642（3 microM）抑制NHE1或KB-R 7943（5 microM）抑制反向模式Na(+)/Ca(2+)交换（NCX）之前和之后，对由此产生的收缩力的即时和缓慢增加（第一阶段和SFR）以及Ca(2+)、Na(+)或pH(i)的变化进行量化。
在Tyrode液组（n = 21）和HEPES组（n = 22）中，第一阶段收缩力增加至约160%，随后在SFR期间进一步增加至约205%。SFR伴随着水母发光蛋白光瞬变增加21%（n = 4；相对于第一阶段进行归一化）和Na(+)增加约3 mM（n = 4 - 7）。SFR还与pH(i)升高有关。然而，这种升高是延迟的，并且仅在SFR达到最大值后才显著。HEPES组中延迟的pH(i)升高比Tyrode液组更大。HOE 642和/或KB-R 7943使SFR降低约30 - 40%。此外，HOE 642使拉伸介导的Na(+)升高降低72%以及延迟的碱化作用。
数据与以下假设一致，即SFR是由于通过NCX的通量改变继发的Ca(2+)增加所致，部分原因是由NHE1介导的Na(+)增加。
