Mobilisation of store Ca2+ activates tyrosine hydroxylase in bovine adrenal chromaffin cells

Full text: Many receptor agonists are able to activate tyrosine hydroxylase (TOH) in bovine adrenal chromaffin cells. The majority of these are dependent on extracellular Ca2+ for this action. Entry of extracellular Ca2+ through voltage-operated Ca2+ channels is very effective at activating TOH. The contribution of the intracellular Ca2+ stores to TOH activation however is not known. Previous studies have shown that mobilisation of intracellular Ca2+ stores is effective at increasing phosphorylation of TOH, but its effect on TOH activity has not been studied. Therefore, in the present study, the effect of mobilisation of store Ca2+ on TOH activity was investigated using primary cultures of bovine adrenal chromaffin cells. Cells were prepared from abattoir tissue and cultured for 3-6 days. TOH activity was determined over 10 minutes, measuring the 14CO2 produced following the hydroxylation and rapid decarboxylation of 14C-tyrosine offered to intact cells. Caffeine increased TOH activity in a concentration-dependent manner with a maximum response of 100% increase at 20mM. This effect was not due to osmolarity since 20mM sucrose had no effect.Nor was it due to inhibition of phosphodiesterases, since the effect of caffeine was still seen in the presence of 1mM IBMX. However,caffeine-induced TOH activation was substantially reduced in the absence of extracellular Ca2+. The results suggest that TOH activity can be increased by mobilising intracellular Ca2+ stores, but that this effect involves extracellular Ca2+ influx, possibly through store-operated channels. Copyright (2001) Australian Neuroscience Society