Increases in intracellular calcium ion concentration during depolarization of cultured embryonic Xenopus spinal neurones

Abstract

1. Changes in intracellular Ca2+ ion concentrations ([Ca2+]i) during potassium-induced depolarizations were studied in cultured embryonic Xenopus spinal neurones using the Ca(2+)-sensitive dye Fura-2 and quantitative fluorescence microscopy. 2. Membrane voltages attained during exposure to bath solutions containing 3, 10, 20, 30, 40 and 50 mM-K+ were determined under current clamp. In 3 mM-K(+)-containing solution (normal saline), the resting potential was -65 mV. The threshold voltage required to observe a measurable rise in [Ca2+]i was -40 mV (external potassium concentration [K+]o = 20 mM). The depolarization-induced [Ca2+]i signal had two components: a non-relaxing component, and, at voltages positive to -40 mV, an additional transient component on the rising phase that decayed over tens of seconds. There was substantial variability in the magnitudes of resting and voltage-induced changes in [Ca2+]i, but [Ca2+]i responses were qualitatively consistent between neurons of similar ages. 3. External potassium (K+o)-induced increases in [Ca2+]i were spatially non-homogeneous. The largest increases were seen in the nucleus, near the base of a major neurite, and in growth cones. Increases occurred more rapidly in neurites and growth cones than in somas. T-type and high-voltage-activated (HVA) channels appeared to be present in all cell regions. 4. Increases in [Ca2+]i evoked by 50 mM-K+ (depolarization to approximately -15 mV) were sensitive to treatments demonstrated to inhibit Ca2+ currents in these cells (T-type, HVA-relaxing and HVA-sustained), including Ni2+ (200 microM), Metenkephalin (17.5 microM), and omega-conotoxin (omega-CgTx; 5.5 microM). [Ca2+]i increases were reduced by caffeine (10 mM) and ryanodine (10-100 microM), agents that affect Ca2+ release from intracellular stores. 5. A sustained increase in [Ca2+]i observed at approximately -40 mV ([K+]o = 20 mM) was investigated in greater detail. Concentrations of Ni2+ sufficient to block T-type Ca2+ current slowed but did not block the rise in [Ca2+]i induced by 20 mM-K+. Met-enkephalin did not affect the [Ca2+]i response. omega-CgTx reduced the amplitude of the [Ca2+]i response, but did not eliminate the sustained component. Verapamil (100 microM), caffeine and ryanodine differentially reduced the sustained component as compared to the initial rising phase. These observations suggest that the rising phase was due to Ca2+ influx through T-type and other Ca2+ channels, and that the sustained phase was differentially sensitive to inhibition of internal Ca2+ release.(ABSTRACT TRUNCATED AT 400 WORDS)