Mechanism of allosteric regulation of the rod cGMP phosphodiesterase activity by the helical domain of transducin alpha subunit

The G protein alpha subunit (Galpha) is composed of two distinct folding domains: a GTP-binding Ras-like domain and an alpha helical domain (HD). We have recently reported that the helical domain (HDt) of the vertebrate visual transducin alpha subunit (Galphat) synergizes activation of retinal cyclic GMP phosphodiesterase (PDE) by activated Galphat (Liu, W., and Northup, J. K., (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 12878-12883). Here, we examine the molecular basis for this HD-based signaling regulation, and we provide a new model for the activation of the target effector. The HD proteins derived from visual transducin or taste gustducin alpha subunits, but no other Galpha HD proteins, each attenuate the PDE catalytic core (Palphabeta) and synergize Galphat stimulation of the holoPDE (Palphabetagamma2) with similar apparent affinities. The data from studies of both HDt-mediated attenuation and stimulation indicate that the HDt and the PDE inhibitory subunit (Pgamma) interact with PDE at independent sites and that Palphabeta contains the binding sites for HD. The saturation of both processes by HDt displays positive cooperativity with Hill coefficients of 1.5 for the attenuation of Palphabeta activity and 2.1 for synergism of holoPDE activation. Our data suggest the that Galphat-HDt regulates PDE by allosterically decreasing the affinity of Palphabeta for Pgamma and thus simultaneously facilitating the interaction of the activated Galphat-Ras-like domain with Pgamma. Thus, we propose a new model for the high efficiency of PDE activation as well as deactivation, and, overall, a novel mechanism for controlling fidelity, sensitivity, and efficacy of G protein signaling.     

Mesencephalic lesions resulting in normophagia, reduced weight and altered metabolism

The effects of bilateral lesions of the ventral noradrenergic bundle (VNA) were studied in male rats. In contrast to data reported by others, hyperphagia and obesity were not observed following VNA lesions. Indeed, except for a depression during the first three postoperative days, food intake (FI) of the VNA lesioned animals (VNAL) was normal. Interestingly, the body weight (BW) of the VNAL was significantly reduced compared to the controls, and a pair feeding study indicated that this depression of BW was not due to their FI. Computation of FI per metabolic size showed that the VNAL actually had a significantly increased FI compared to the controls. After a two day fast the VNAL lost more metabolic size than controls and upon refeeding they defended their pre-fast BW. The VNAL rats showed normal body composition and circulating glucose, insulin and prolactin. They had reduced free fatty acids, triglycerides, growth hormone and body length. The data suggest that the mesencephalon influences BW set point, some metabolites and possibly overall metabolism.     

The effect of calf lymph and bovine red blood cells on in vitro cultivation of Theileria parva-infected lymphoid cells

Calf lymph introduced to cultures of Theileria parva-infected lymphoid cells caused a stimulation of both cells and parasites resulting in the formation of rapidly dividing cells and the developmental stages of the parasite. The presence of piroplasms was confirmed when washed bovine red blood corpuscles were added to the culture     

The effects of alcohol withdrawal and acute doses of alcohol on the acid-base balance in mice and rats

While respiratory alkalosis has been reported to occur in human alcoholics during the withdrawal period, and may causally contribute to the expression of the withdrawal syndrome, we found no evidence of this in either alcohol physically-dependent mice (DBA/2J) or rats (Sprague-Dawley) as reflected in blood pH, plasma pCO2, or intracellular brain pH relative to pair-fed control (no alcohol) animals. The inhalation of atmospheres high in CO2 (5% and 15%), which would be expected to reverse a respiratory alkalotic state, had no effect on the expression of the withdrawal syndrome in alcohol-dependent mice. These results indicate that a withdrawal syndrome of considerable intensity can develop in mice and rats in the absence of an alkalotic state.