Question:

Hypertension can cause cardiac hypertrophy. We hypothesized that changes in cardiomyocyte and nucleus structure and nuclear Ca signaling occur early in hypertension contributing to the initiation and progression of hypertrophy. To test this we studied subcellular Ca signaling in the cyto- and nucleoplasm of ventricular myocytes from young hypertensive rats.

Methods:

Isolated ventricular myocytes from 12-14 weeks old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats (controls) were electrically stimulated (1Hz, RT). Ca transients (CaTs) were recorded by whole cell epifluorescence (5µM fura-2/AM) or line scan confocal microscopy (8µM fluo-4/AM). Perinuclear Ca stores were imaged with Mag-Fluo-4/AM (12µM). SR and perinuclear Ca loads were evaluated using rapid application of caffeine (10mM).

Results:

Left ventricle (LV), ventricular myocytes and nuclei were enlarged in SHR (cell: SHR (n=175) 3866±101µm2 vs WKY (n=266) 3400±111µm2; P<0.01; nucleus: SHR (n=227) 53.8±1.2µm2 vs WKY (n=313) 44.2±0.7µm2; P<0.01) with a significantly increased nucleus-to-cell area ratio. The number of nuclear envelope invaginations was not different between the two groups of rats (SHR (n=173) 3.4±0.1 versus WKY (n=152) 3.3±0.1 invaginations per nucleus). Whole cell CaTs were increased in SHR (systolic fura-2 ratio: SHR (n=11) 1.29±0.07 vs WKY (n=10) 1.03±0.08; P<0.05), with no differences in diastolic and resting Ca levels. Confocal imaging indicated that SHR myocytes (n=46) had bigger cyto- (dF/Frest: SHR 8.34±0.33 vs WKY 6.32±0.25; P<0.01) and nucleoplasmic (dF/Frest: SHR 5.91±0.34 vs WKY 3.32±0.18; P<0.01) CaTs than WKY myocytes (n=57) with an increased nucleoplasmic-to-cytoplasmic CaT amplitude ratio (SHR 0.70±0.04 vs WKY 0.51±0.01; P<0.01) and significantly faster CaT decay both in the cyto- and nucleoplasm. Calcium propagation from the subnucleolemmal to the central nuclear region was faster in SHR (87±11µm/s; n=41) than in WKY nuclei (56±8µm/s; n=46; P<0.05). Caffeine-evoked cyto- and nucleoplasmic CaTs (dF/Frest) were bigger in SHR (n=43) than in WKY (n=31) (cytoplasm: SHR 10.77±0.34 vs WKY 9.63±0.46; nucleoplasm: SHR 10.98±0.37 vs WKY 9.15±0.50; P<0.05) suggesting increased SR and perinuclear Ca load in SHR myocytes.

Conclusions:

Our results show that cardiomyocyte hypertrophy and increases in nuclear size and Ca signaling occur early in hypertension and might contribute to the initiation and progression of hypertrophy.