Secretory Potential and Ionic Transport in the Posterior Silk Glands of Bombyx Mori
Authors: Nakagaki I. , Sasaki S. 
Published: April 2021

ABSTRACT
The concentrations of Na, Mg, P, S, Cl, K and Ca in the cytoplasm and lumen of the posterior silk gland cells of Bombyx mori were measured by X-ray microprobe analysis of freeze-dried thin sections. The basal and luminal membrane potentials of the gland cells were measured using microelectrode techniques. The input resistance of the luminal plasma membrane was simultaneously measured by injecting electric current via an intracellular microelectrode. The basolateral membrane potential was −47 ± l·8mV (S.E.) (N = 46), and the glands exhibited lumen-negative voltages of −6± 0·1 mV (S.E.) (N = 40) in the normal state. Increasing the extracellular K+ concentration depolarized the basolateral membrane potential, whereas the membrane potential hyperpolarized when Cl− concentration in the extracellular fluid was increased. There were no significant effects on the membrane potential when Na+, Mg2+ and Ca2+ concentrations in the extracellular fluid were changed. The representative X-ray spectra showed high K and phosphorus peaks, and low Cl and Mg peaks in the cytoplasm of the normal posterior silk gland cells. The normal glandular lumen showed relatively high K, and low Cl, sulphur, Ca and Mg peaks. Quantitative microprobe values were, for the cytoplasm (mmol kg−1 wet mass, N =30) Na, 5; Mg, 14; phosphorus, 168; sulphur, 16; Cl, 12; K, 168; Ca, 0 5; and for the lumen (N= 10) Na, 3; Mg, 25; phosphorus, 42; sulphur, 24; Cl, 38; K, 133; Ca, 9·4 in the normal glands. The basal plasma membrane potential was hyperpolarized by 7 mV after stimulation with 5×10− 5 mmol l−1 5-hydroxytryptamine (5-HT). Microprobe values for the cytoplasm were (mmol kg−1 wet mass, N =15) Na, 4; Mg, 13; phosphorus, 160; sulphur, 17; Cl, 8; K, 187; Ca, 0·6 in the stimulated glands. The cytoplasmic [K] increased after stimulation with 5-HT. The basal membrane potential of the gland cells was depolarized by 3 mV after application of a juvenoid, methoprene (10− 5moll−1). X-ray microprobe values for the cytoplasm were (mmol kg−1 wet mass, N = 15) Na, 7; Mg, 11; P, 170; S, 14; 0,23; K, 130; Ca, 3·4 in the treated glands. The cytoplasmic [Ca] and [Cl] increased, while the [K] decreased with methoprene stimulation. The luminal membrane potential of the gland cells was depolarized by 8 mV and a simultaneous decrease of luminal membrane resistance was apparent after stimulation with an anti-microfilament reagent, cytochalasin D (2×10−6mol l−1). X-ray microprobe values for the cytoplasm became (mmol kg−1 wet mass, N = 15) Na, 10; Mg, 28; P, 194; S, 22; Cl, 24; K, 148; Ca, 1·2; and for the lumen (N = 15) Na, 14; Mg, 13; phosphorus, 31; sulphur, 30; Cl, 92; K, 122; Ca, 1-1 in the stimulated glands. The cytoplasmic [Ca] and [Cl] increased and [K] decreased, whereas the luminal [Cl] and [sulphur] increased and [Ca] and [Mg] decreased after cytochalasin D stimulation. The reaction products of adenosine triphosphatase activity were found on the luminal and lateral plasma membranes of the posterior silk gland cells. The possible routes of ion transport into the lumen are discussed.