Induction of division synchrony in Tetrahymena pyriformis by a single hypoxic shock. Its use in elucidating control of the cell cycle by adenosine 3':5'-monophosphate

Cyclic AMP formation in Tetrahymena pyriformis is controlled by a K(+)-conductance

Responses of the cAMP generating system of Tetrahymena to changes in the concentrations of external [K+] or [Ca2+] ions were examined. When Tetrahymena are equilibrated in high [K+] buffers, intracellular levels of cAMP decreased to 40% within 2 h. Hyperpolarization of the cells by dilution of external [K+] to one-eighth of its original concentration instantly stimulated intracellular cAMP formation. Manipulations of the K+ resting conductance of Tetrahymena by equilibration in buffers of different K+ content greatly affected the responsivity of the adenylyl cyclase. Hyperpolarization of the cell by addition of Ca2+ also resulted in a rapid generation of cAMP. Blockade of K+ conductances by the K+ channel blockers tetraethylammonium, quinine, and Cs+, dose-dependently inhibited hyperpolarization-stimulated cAMP formation. The data indicate that a hyperpolarization-activated K+ current is directly coupled to adenylyl cyclase regulation.

Cell surface interactions in conjugation: Tetrahymena ciliary membrane vesicles

Tetrahymena ciliary membrane vesicles are shown to interact with preconjugant cells in a mating type-specific way. When cells are treated with vesicles of a different mating type before mixing for conjugation, cell pairing is enhanced, and the normal prepairing period is partially eliminated. This enhancement is mating type specific since it is not observed after pretreatment of cells with vesicles of their own mating type. In contrast, when vesicles are added at the time of mixing of two starved cultures, cell pairing is delayed in a concentration-dependent manner. By varying the conditions, we demonstrated enhancement or inhibition, or both. These results are interpreted in terms of two independent interactions of cells with vesicles. We suggest that first, vesicles substitute for another cell in cell-cell prepairing interaction and second, vesicles compete for adhesion sites produced during the prepairing period. Finally, the data presented are summarized within a speculative framework that calls attention to potential analogies with hormone-receptor signaling in mammalian cells.

Cell surface interactions in conjugation: Tetrahymena ciliary membrane vesicles

Tetrahymena ciliary membrane vesicles are shown to interact with preconjugant cells in a mating type-specific way. When cells are treated with vesicles of a different mating type before mixing for conjugation, cell pairing is enhanced, and the normal prepairing period is partially eliminated. This enhancement is mating type specific since it is not observed after pretreatment of cells with vesicles of their own mating type. In contrast, when vesicles are added at the time of mixing of two starved cultures, cell pairing is delayed in a concentration-dependent manner. By varying the conditions, we demonstrated enhancement or inhibition, or both. These results are interpreted in terms of two independent interactions of cells with vesicles. We suggest that first, vesicles substitute for another cell in cell-cell prepairing interaction and second, vesicles compete for adhesion sites produced during the prepairing period. Finally, the data presented are summarized within a speculative framework that calls attention to potential analogies with hormone-receptor signaling in mammalian cells.

Cell cycle specific fluctuations of adenosine 3',5' -monophosphate and prostaglandin binding in synchronized mastocytoma P-815 cells

Endogenous adenosine 3',5' -monophosphate (cAMP) levels in mastocytoma P-815 cells, synchronized either at the G1/S transition by amethopterin- or double thymidine-block or in mitosis by colcemid block, were highest during late S and early G2 phases and lowest during mitosis. These cell cycle-dependent changes in cAMP levels were largely accounted for by changes in adenylate cyclase and phosphodiesterase activities. Similar fluctuations occurred simultaneously with specific prostaglandin E1 (PGE1) binding, histidine decarboxylase activity, histamine content, and [35S]SO-2(4) incorporation into glycosaminoglycans of the cells. In addition, endogenous levels of the E group of prostaglandins (PGEs) and "14C]carachiodonic acid incorporations into PGE, phosphatidylcholine and phosphatidylinositol also exhibited fluctuation patterns similar to that of cAMP levels. Since cAMP levels still fluctuated in a serum-depleted medium where DNA synthesis and cell division were inhibited, endogeneous levels of prostaglandin and cAMP appeared not to be regulated solely by serum factor(s). Exposure of cells at G1/S transition to 1-methyl-3-isobutylxanthine (MIX) resulted in 10-fold elevation of cAMP levels throughout the cell cycle without affecting DNA synthesis. On the other hand, PGE1 and/or MIX added at late S phase elevated cAMP levels, prolonged C2 phase and retarded the cell division, but these agents added at the beginning of mitosis elevated cAMP levels without affecting the cell division. These results suggest that prostaglandin newly synthesized by the increased metabolism of phospholipids promote the cAMP synthesis via their binding to the receptors and thereby control the division and phenotypic expression of mastocytoma P-815 cells.

Factors modulating the response of a porcine renal tubular cell line to calcitonin and antidiuretic hormone

A cell line (LLC-PK1) isolated from porcine kidney increases 3'5'-cyclic adenosine monophosphate (cAMP) content when incubated with salmon calcitonin (SCT) or antidiuretic hormone (ADH). We have examined several factors which modulate the hormone-induced changes in cAMP levels in these cells. Preincubation with increasing concentrations of SCT results in a dose-dependent decrease in cAMP levels in cells retested with this hormone. Addition of 3-isobutyl-l-methylxanthine (IBMX) to cells preincubated with SCT results in 15--30-fold increases in cAMP levels compared to cells preincubated without this hormone. These observations suggest that the decrease in SCT-induced cAMP response in cells pretreated with this hormone is related at least in part to stimulation of phosphodiesterase activity. Preincubation with ADH does not affect subsequent cAMP response to either ADH or SCT, suggesting that these hormones interact with different cell surface receptors. Cell cycle and plating density also affect cAMP levels. cAMP content per cell increases with increasing cell density, which is associated with an increase in the SCT-induced cAMP response. These studies illustrate that factors other than receptor occupancy modulate cAMP responses of these cells to specific hormones.

Intracellular pH changes during the cell cycle in Tetrahymena

The equilibrium distribution of 5,5-dimethyloxazoladine 2,4-dione (DMO) between intra- and extracellular volume was used to estimate intracellular pH (pHi) in Tetrahymena pyiformis. In control experiments, DMO was found to equilibrate rapidly in response to a pH gradient. Under normal growth conditions, pHi was constant over a finite range of external pH, being maintained near pH 7.1 over the external pH range 5.2 to 7.3. This same range of external pH was also optimal for growth. pHi was monitored during the cell cycle of a synchronous population of T. pyriformis GL. The cells were synchronized either by starvation/refeeding or heat shock. Under both conditions, there were two alkaline shifts of approximately 0.4 pH units per cell cycle. These shifts in pH retained a constant remporal relationship to S phase and were not affected by changes in the time, duration, or magnitude of cytokinesis.