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[Effects of microtubule depolymerization on spontaneous beating and action potential of cardiac myocytes in rats and its mechanism].

OBJECTIVE
To explore the effects of microtubule depolymerization (MD) on the spontaneous beating rate, action potential (AP), and oxygen consumption of cardiac myocytes in rats and its mechanism.


METHODS
One-hundred and eighty neonatal SD rats divided into 12 batches were used in the experiment, and 15 rats in each batch were sacrificed for the isolation and culture of cardiac myocytes after the heart tissues were harvested. The cardiac myocytes were respectively inoculated in one 12-well plate filled with 6 round cover slips, one 12-well plate filled with 6 square cover slips, two cell culture flasks, and two cell culture dishes. After routine culture for three days, the cardiac myocytes from all the containers were divided into normal control group (NC, routinely cultured with 3 mL DMEM/F12 solution rewarmed at 37 °C for 3 h) and group MD (routinely cultured with 3 mL DMEM/F12 solution rewarmed at 37 ° and containing 8 µmol/L colchicine for 3 h) according to the random number table, with 3 holes, 1 flask, or 1 dish in each group. The morphological changes in microtubules were observed with confocal laser scanning microscope after immunofluorescent staining. The content of polymerized or dissociative α-tubulin was determined by Western blotting. Spontaneous beating rate of the cells was observed and calculated under inverted microscope. Dissolved oxygen concentration of DMEM/F12 solution containing cardiac myocytes was determined by oxygen microelectrode system before and after the addition of colchicine. Additionally, dissolved oxygen concentration of DMEM/F12 solution and colchicine + DMEM/F12 solution was determined. The whole-cell patch-clamp technique was used to record AP, delayed rectifier K+ current (I(K)), and L-type Ca2+ current (I(Ca-L)) in cardiac myocytes; current density-voltage (I-V) curves were drawn based on the traces. Data were processed with independent or paired samples t-test.


RESULTS
(1) In group NC, microtubules of cardiac myocytes were around the nucleus in radial distribution with intact and clear linear tubiform structure. The microtubules in group MD were observed in dispersive distribution with damaged structure and rough linear tubiform structure. (2) In group MD, the content of dissociative α-tubulin of cells (0.61 ± 0.03) was obviously higher than that in group NC (0.46 ± 0.03, t = -6.99, P < 0.05), while the content of polymerized α-tubulin (0.57 ± 0.04) was significantly lower than that in group NC (0.88 ± 0.04, t = 9.09, P < 0.05). (3) Spontaneous beating rate of cells was (59 ± 8) times per min in group MD, which was distinctly higher than that in group NC [(41 ± 7) times per min, t = 5.62, P < 0.01]. (4) Dissolved oxygen concentration of DMEM/F12 solution containing cardiac myocytes was (138.4 ± 2.5) µmol/L, and it was reduced to (121.7 ± 3.6) µmol/L after the addition of colchicine ( t = 26.31, P < 0.05). There was no obvious difference in dissolved oxygen concentration between DMEM/F12 solution and colchicine + DMEM/F12 solution (t = 0.72, P > 0.05). (5) Compared with that of group NC, AP morphology of cells in group MD changed significantly, with unobvious repolarization plateau phase and shorter action potential duration (APD). The APD20, APD50, and APD90 were respectively (36.2 ± 3.8), (73.7 ± 5.7), and (115.1 ± 8.0) ms in group MD, which were significantly shorter than those of group NC [(40.2 ± 2.3), (121.4 ± 7.0), and (169.4 ± 5.6) ms, with t values respectively 2.61, 15.88, and 16.75, P values below 0.05]. (6) Compared with that of group NC, the I-V curve of I(K) of cells in group MD moved up with higher current density under each test voltage (0 to 40 mV) after activation ( with t values from 2. 70 to 3. 76, P values below 0.05) . (7) There was not much alteration in current density of I(Ca-L) under each test voltage (-30 to 50 mV) between 2 groups (with t values from -1.57 to 1.66, P values above 0.05), and their I-V curves were nearly overlapped.


CONCLUSIONS
After MD, the I(K) is enhanced without obvious change in I(Ca-L), making AP repolarization faster and APD shortened. Then the rapid spontaneous beating rate increases oxygen consumption of cardiac myocytes of rats.