Regulation and metabolic role of phospholipase D activity in human thyroid and cultured dog thyrocytes

P2X7 receptor stimulation of membrane internalization in a thyrocyte cell line

Using fluorescent membrane markers, we have previously shown that extracellular ATP stimulates both exocytosis and membrane internalization in the Fisher rat thyroid cell line FRTL. In this study, we examine the actions of ATP using whole-cell recording conditions that favor stimulation of membrane internalization. ATP stimulation of the P2X(7) receptor activated a reversible, Ca(2+)-permeable, cation conductance that slowly increased in size without changes in ion selectivity. ATP also induced a delayed irreversible decrease in cell capacitance (C(m)) that was equivalent to an 8% decrease in membrane surface area. Addition of guanosine 5'-0-2-thiodiphosphate to the pipette solution inhibited the ATP-induced decrease in C(m) without affecting channel activation. The effects of ATP on membrane conductance were mimicked by 2',3'-O-(4-benzoylbenzoyl)-ATP, but not by UTP, adenosine, or 2-methylthio-ATP, and were inhibited by pyridoxal phosphate-6-azophenyl-2'4'-disulfonic acid, adenosine 5'-triphosphate-2'3'-dialdehyde, and Cu(2+). The capacitance decrease persisted in Na(+)-, Ca(2+)- and Cl(-)-free external saline or with Ca(2+)-free pipette solution. It is concluded that ATP activation of the inotropic P2X(7) receptor stimulates membrane internalization by a mechanism that involves intracellular GTP, but does not require internal Ca(2+) or influx of Na(+) or Ca(2+) through the receptor-gated channel.

A P2X7 receptor stimulates plasma membrane trafficking in the FRTL rat thyrocyte cell line

Thyroid cells express a variety of P2Y and P2X purinergic receptor subtypes. G protein-coupled P2Y receptors influence a wide variety of thyrocyte-specific functions; however, functional P2X receptor-gated channels have not been observed. In this study, we used whole cell patch-clamp recording and fluorescence imaging of the plasma membrane marker FM1-43 to examine the effects of extracellular ATP on membrane permeability and trafficking in the Fisher rat thyroid cell line FRTL. We found a cation-selective current that was gated by ATP and 2',3'-O-(4-benzoylbenzoyl)-ATP but not by UTP. The ATP-evoked currents were inhibited by pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, adenosine 5'-triphosphate-2',3'-dialdehyde, 100 microM Zn(2+), and 50 microM Cu(2+). Fluorescence imaging revealed pronounced, temperature-sensitive stimulation of exocytosis and membrane internalization by ATP with the same pharmacological profile as observed for activation of current. The EC(50) for ATP stimulation of internalization was 440 microM in saline containing 2 mM Ca(2+) and 2 mM Mg(2+), and 33 microM in low-Mg(2+), nominally Ca(2+)-free saline. Overall, the results are most consistent with activation of a P2X(7) receptor by ATP(4-). However, low permeability to N-methyl-d-glucamine(+) and the propidium cation YO-PRO-1 indicates absence of the cytolytic pore that often accompanies P2X(7) receptor activation. ATP stimulation of internalization occurs in Na(+)-free, Ca(2+)-free, or low-Mg(2+) saline and therefore does not depend on cation influx through the ATP-gated channel. We conclude that ATP activation of a P2X(7) receptor stimulates membrane internalization in FRTL cells via a transduction pathway that does not depend on cation influx.