EFFECTS OF LOWERING EXTERNAL NA-CELLS - MECHANISM OF PERIODICITY OF SPIKE-BURSTS( CONCENTRATION ON CYTOPLASMIC PH AND CA2+ CONCENTRATION INMOUSE PANCREATIC BETA)
M. Hattori et al., EFFECTS OF LOWERING EXTERNAL NA-CELLS - MECHANISM OF PERIODICITY OF SPIKE-BURSTS( CONCENTRATION ON CYTOPLASMIC PH AND CA2+ CONCENTRATION INMOUSE PANCREATIC BETA), Japanese Journal of Physiology, 44(3), 1994, pp. 283-293
The periodic spike-burst response of pancreatic beta-cells varies in d
uration with an increase of external glucose within the range 5-20 mM.
To elucidate the mechanism determining the length of spike-burst, we
studied the low-Na+ induced change in electrical response to glucose w
hich is similar to the change induced by high glucose. Cytoplasmic pH
(pH(i)) and Ca2+ concentration ([Ca2(+)](i)) were measured by the micr
ofluorometric method under normal and low-Na+ conditions in mouse panc
reatic islets. Lowering external Na+ concentration from 135 to 25 mM b
y replacing Na+ with Tris(+) induced progressive alkalinization in isl
et cells in the presence of 11.1 mM glucose. In contrast, reduction of
external Na+ by replacement with Li+ caused intracellular acidificati
on. Both manipulations described above caused a marked increase in [Ca
2(+)](i), suggesting the presence of Na+/Ca2+-antiport activity. Altho
ugh the change in pH(i) induced by decreasing external Na+ varied in d
irection depending on the species of cations used for replacing Na+, t
he pattern of electrical activity consistently changed from the spike-
burst type to the continuous spike-generation type without regard for
the difference in species of cations replacing Na+. These findings lea
d to the following hypothesis: A decrease in Na+ influx could cause a
decrease in ATP-consumption by Na+/K+-pumps that prevents the fall of
intracellular ATP concentration. The resultant continuation of high co
ncentrations of intracellular ATP may be responsible for the abolishme
nt of the silent phase.