Biological characteristics of the globular substance in the otoconial membrane of the guinea pig.

Biological characteristics of the globular substance, which is considered to be a precursor of otoconia, were investigated by means of confocal laser scanning microscopy. The shape of the globular substance was a complete sphere, 3-10 microns in diameter. Its surface stained positively with both rhodamine 123 and DiOC6(3), implying similarity to intracellular organelles, whereas no fluorescence was seen when stained with chlortetracycline, a membrane-associated Ca2+ dye. Meanwhile, this substance showed very little affinity for six kinds of lectins, indicating the lack of a surface structure of carbohydrates. The fluorescence of fluo-3 in the globular substance increased markedly after the application of ionomycin. But this was completely inhibited by the depletion of external Ca2+. This reaction suggests that the surface of the globular substance exhibits characteristics of a biological membrane and that the influx of external Ca2+ occurs through membrane-combined ionomycin. Internal free Ca2+ concentration varied from 1.1 x 10(-9) to 1.6 x 10(-4) M, the geometric mean being 3.3 x 10(-7) M, which is higher than normal resting level of intracellular Ca2+ concentration but lower than the calcium content of the globular substance estimated by X-ray microanalysis in previous studies.