Abstract. Nuclear pore complexes (NPCs) prepared from Xenopus laevis oocyte nuclear envelopes were studied in "intact" form (i.e., unexposed to detergent) and after detergent treatment by a combination of conventional transmission electron microscopy (CTEM) and quantitative scanning transmission electron microscopy (STEM). In correlation-averaged CTEM pictures of negatively stained intact NPCs and of distinct NPC components (i.e., "rings" "spoke" complexes, and "plug-spoke" complexes), several fine structural features arranged with octagonal symmetry about a central axis could reproducibly be identified. STEM micrographs of unstained/freeze-dried intact NPCs as well as of their components yielded comparable but less distinct features. Mass determination by STEM revealed the following molecular masses: intact NPC with plug, 124 + 11 MD; intact NPC without plug, 112 + 11 MD; heavy ring, 32 + 5 MD; light ring, 21 ± 4 MD; plug-spoke complex, 66 ± 8 MD; and spoke complex, 52 ± 3 MD. Based on these combined CTEM and STEM data, a three-dimensional model of the NPC exhibiting eightfold centrosymmetry about an axis perpendicular to the plane of the nuclear envelope but asymmetric along this axis is proposed. This structural polarity of the NPC across the nuclear envelope is in accord with its well-documented functional polarity facilitating mediated nucleocytoplasmic exchange of molecules and particles.

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The composition and pH of atrial-specific granules of rat heart were determined by electron probe x-ray microanalysis and fluorescence microscopy, respectively. The high (75 mmol/kg of dry weight) calcium content and higher than cytoplasmic concentration of chloride in atrial-specific granules were visualized in high-resolution x-ray maps. The C1-content of granules and cytoplasm decreased and a bromide gradient (granule > cytoplasm) was established during incubation in low-chloride, NaBr-containing solutions.Scanning confocal fluorescence light microscopy of live atria incubated with acridine orange demonstrated dye accumulation, indicative of low intragranular pH. We conclude that the granules represent a hitherto unrecognized intracellular store of cardiac calcium and can develop and maintain an anion gradient, presumably through cotransport by means of a proton-pumping ATPase.Atrial-specific granules (ASGs), described nearly three decades ago and present in the heart of most vertebrates (1, 2), contain precursors of a family of vasodilator and natriuretic polypeptides, collectively called atrial natriuretic factor (ANF) (3-7). The heart can function as an endocrine organ, releasing ANF in response to changes in atrial distending pressure and other stimuli (8). We now show that ASGs share other properties of endocrine secretory granules, such as an acidic interior and the ability to establish a sizable anion gradient. We also show that these organelles have a high Ca content and are a hitherto unknown store of intracellular Ca in atrial myocardium. Based on the known sequences ofANF prohormone (9, 10) and the Ca-binding protein calsequestrin (11), we suggest that the high Ca content of ASGs is probably due to binding to grouped acidic residues in the aminoterminal sequence of the prohormone. METHODS AND MATERIALSMale Wistar rats (250-300 g of body weight) were killed by cervical dislocation, and the hearts were quickly excised and placed in a well-oxygenated modified Krebs/bicarbonate solution (pH 7.4) at 32°C containing (mM) NaCl, 125; KCI, 4.7; KH2PO4, 1.2; MgSO4, 1.2; NaHCO3, 18.7; CaCl2, 1.2; dextrose, 5.6. The right atrium was everted over a 1.5-mm (diameter) silver pin and tied at the base. Tension was not monitored. The atria in Krebs solution generally beat spontaneously up to the point of freezing in either systole or diastole. To determine whether ASGs can establish an anion gradient de novo, atria were incubated for 15 min at 32°C in a solution (pH 7.4) containing (mM) sodium methanesulfonate, 105; NaBr, 20; potassium methanesulfonate, 4.7;

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