8-diethylamino-octyl-3,4,5-trimethoxybenzoate, a calcium store blocker, increases calcium influx, inhibits α-1 adrenergic receptor calcium mobilization, and alters iodide transport in FRTL-5 rat thyroid cells

8 - Diethylamino - octyl - 3,4,5 - trimethoxybenzoate (TMB-8) is known to inhibit mobilization of calcium from intracellular stores but, more recently, other cellular effects have been described. In the present study, the effects of TMB-8 on cytosolic free calcium [Ca²⁺]_i levels in FRTL-5 rat thyroid cells were determined using the fluorescent dye, Indo-1. TMB-8 increased [Ca²⁺]_i in a dose-dependent manner, with a maximum rise from 120 ± 7 nM to 229 ± 16 nM (90 ± 5% increase) at 5 × 10^-4 M. This effect was considerably reduced in Ca²⁺ free buffer, demonstrating a dependency upon extracellular calcium influx but not upon membrane potential and which did not involve the Na⁺/Ca²⁺ exchanger. In Ca²⁺ free buffer TMB-8, at a dose which did not affect [Ca²⁺]_i, completely prevented norepinephrine (10^-5 M) from mobilizing intracellular Ca²⁺. To determine whether TMB-8 affected differentiated functions, iodide uptake and efflux studies were performed with ¹²⁵I. TMB-8 (10^-4 M) inhibited iodide uptake by approximately 40% without affecting efflux. At 10^-3 M TMB-8, efflux was also enhanced. These studies demonstrate that TMB-8 has at least two effects on [Ca²⁺]_i, promoting calcium influx and preventing α-1 adrenergic mobilization from intracellular stores. TMB-8 also has multiple effects on ¹²⁵I transport, both inhibiting influx and increasing efflux. The results emphasize the importance of characterizing the behavior of this compound in any cell system before using it as a biological probe.