To learn more of the role of calcium in the regulation of melanogenesis, we have used direct manipulation of medium calcium and pharmacological modulation of intracellular calcium to examine the consequences on unstimulated and cyclic AMP elevated tyrosinase activity and melanin synthesis and distribution in B16 melanoma cells. In unstimulated cells, calcium is clearly inhibitory to tyrosinase activity. However, in cells stimulated with cAMP-elevating agents the requirement for extracellular calcium was changed such that cells required a minimum of 0.4-0.6 mmol medium calcium for maximum tyrosinase response to these agents. Paradoxically, pharmacologically increasing intracellular calcium in cAMP-stimulated cells with ionophore inhibited tyrosinase activity, and the calcium-lowering agent TMB8 and the calcium channel blocker verapamil both stimulated tyrosinase activity. When melanin synthesis was measured in cAMP-stimulated cells, TMB8 was found to significantly increase the sensitivity and the maximum melanogenic response to alpha-MSH, suggesting the presence of at least one level of endogenous calcium inhibitory control operative in these cells. In addition, TMB8 changed the distribution of melanin between the cell and the medium such that, in the presence of alpha-MSH and TMB8, significantly more melanin was secreted into the medium. These data suggest that calcium is required for several steps in melanogenesis, having an apparently inhibitory effect on pre-tyrosinase activity in unstimulated cells, but also showing evidence of a positive role in cyclic AMP-stimulated tyrosinase activity, as well as a further possible inhibitory role in melanin movement or secretion.

Diadenosine polyphosphates present in the extracellular environment can, through interaction with appropriate purinoceptors, influence a range of cellular activities. Here we have investigated the nature of the ligand:receptor interactions involved in diadenosine 5',5'''-P1, P4-tetraphosphate (Ap4A)-mediated stimulation of glycogen breakdown in isolated rat liver cells. [2-3H]Ap4A showed specific binding to both intact isolated liver cells and plasma membrane fractions prepared from isolated liver cells. HPLC analysis confirmed that binding was mediated by intact Ap4A and not by potential breakdown products (e.g. ATP, adenosine, etc.). Binding of [2-3H]Ap4A, to isolated liver cell plasma membrane preparations, was successfully displaced by a range of both naturally occurring and synthetic diadenosine polyphospates with the rank order potency Ap4A > or = Ap5A > Ap6A > Ap3A > Ap2A. [2-3H]Ap4A binding was not displaced by P1 effectors but was successfully displaced by a range of P2 effectors with the rank order potency 2-methylthio-ATP > ATP > ATP > or = adenosine 5'-[alpha beta-methylene]triphosphate > adenosine 5'-[beta gamma-methylene]triphospate. These findings are consistent with the interaction of Ap4A with a P2y-like subclass of purinoceptor and are discussed in relation to (1) the known purinoceptor populations in liver cell plasma membranes and (2) observations concerning the binding of diadenosine polyphosphates to purinoceptors in other tissues.

The activity of the cell cycle control protein p34cdc2 is post‐translationally regulated in a variety of cell types. Using anti‐phosphotyrosine antibodies, we find that p34cdc2‐directed tyrosine kinase activity increases at fertilization in sea urchin eggs, leading to a gradual accumulation of phosphotyrosine on p34 during the early part of the cell cycle. Loss of phosphotyrosine from p34 accompanies entry into mitosis and phosphotyrosine reaccumulates as the embryo enters the next cell cycle. A similar pattern is seen when eggs are parthenogenetically activated with ammonium chloride. Tyrosine phosphorylation and phosphorylation/dephosphorylation cycles are suppressed when embryos are treated with the tyrosine kinase inhibitor genistein. On the other hand, a cycle persists when protein synthesis is inhibited with emetine, indicating that it is independent of the synthesis of another class of cell cycle control proteins, the cyclins. Additional experiments with the phorbol ester, phorbol myristate acetate, demonstrate that activating protein synthesis alone in unfertilized eggs does not result in stimulation of p34cdc2 tyrosine kinase activity. Our results indicate that p34 tyrosine phosphorylation cycles are triggered by the fertilization Cai transient. The first cycle is independent of the fertilization pHi signal, confirming that, in sea urchin embryos, the cycle is not tightly coupled to the cycle of cyclin abundance that is a prominent feature of the eukaryotic cell division cycle.

Diadenosine polyphosphates released into the extracellular environment influence a variety of metabolic and other cellular activities in a wide range of target tissues. Here we have studied the impact of these novel nucleotides on gluconeogenesis in isolated rat proximal tubules. Gluconeogenesis was stimulated following exposure of isolated proximal tubules to a range of adenine-containing nucleotides including ADP, ATP, Ap3A, Ap4A, Ap5A and Ap6A. The concentration-dependence of ATP-, Ap3A- and Ap4A-mediated stimulation of gluconeogenesis was similar and was consistent with a role for these agents in the physiological control of renal metabolism. Nucleotide-stimulated gluconeogenesis was diminished in the presence of agents that interfere with phospholipase C activation or intracellular Ca2+ metabolism, indicative of a role for polyphosphoinositide-mediated Ca2+ mobilization in the mechanism of action of ATP, Ap3A and Ap4A. The characteristics of binding of [2-3H]Ap4A to renal plasma-membrane preparations suggest that Ap4A mediates its effects on proximal tubule gluconeogenesis via interaction with P2y-like purinoceptor(s) also recognized by extracellular ATP.

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