Muscarinic K+ channel in the heart. Modal regulation by G protein beta gamma subunits.

The membrane-delimited activation of muscarinic K+ channels by G protein beta gamma subunits plays a prominent role in the inhibitory synaptic transmission in the heart. These channels are thought to be heterotetramers comprised of two homologous subunits, GIRK1 and CIR, both members of the family of inwardly rectifying K+ channels. Here, we demonstrate that muscarinic K+ channels in neonatal rat atrial myocytes exhibit four distinct gating modes. In intact myocytes, after muscarinic receptor activation, the different gating modes were distinguished by differences in both the frequency of channel opening and the mean open time of the channel, which accounted for a 76-fold increase in channel open probability from mode 1 to mode 4. Because of the tetrameric architecture of the channel, the hypothesis that each of the four gating modes reflects binding of a different number of Gbeta gamma subunits to the channel was tested, using recombinant Gbeta1 gamma5. Gbeta1 gamma5 was able to control the equilibrium between the four gating modes of the channel in a manner consistent with binding of Gbeta gamma to four equivalent and independent sites in the protein complex. Surprisingly, however, Gbeta1 gamma5 lacked the ability to stabilize the long open state of the channel that is responsible for the augmentation of the mean open time in modes 3 and 4 after muscarinic receptor stimulation. The modal regulation of muscarinic K+ channel gating by Gbeta gamma provides the atrial cells with at least two major advantages: the ability to filter out small inputs from multiple membrane receptors and yet the ability to create the gradients of information necessary to control the heart rate with great precision.

Turnover of ribosomal proteins in regenerating rat liver after partial hepatectomy.

The rates of synthesis and degradation of ribosomal proteins, prelabelled with [14C]bicarbonate, were determined as an index of the rate of ribosome turnover in regenerating rat liver. The half-life of ribosomes is about 178 and 75 hr in regenerating and normal liver, respectively. The comparison of turnover rates of ribosomal proteins with the corrected values of rRNA, based on re-utilization of nucleotides, suggests that ribosomes are metabolized as a unit in vivo. There is at least 70% overestimation for ribosome half-life when orotate-labelled RNA is used for turnover determinations. The absolute rate of synthesis is estimated as 3925 and 1081 ribosomes/min per cell in 24 hr regenerating and normal rat liver, respectively.

Re-utilization of pyrimidine nucleotides during rat liver regeneration.

The changes in the specific radioactivities of the pool of total acid-soluble uridine nucleotides and of uridine and cytidine components of total cellular and nuclear RNA were monitored in regenerating rat liver for 12 days after partial hepatectomy. Evidence is presented for the re-utilization of pyrimidine nucleotides derived from cytoplasmic RNA degradation for the synthesis of new RNA. The extent of recycling was assessed and the true rate of rRNA turnover determined more accurately. The reutilization of the uridine components of RNA was 7.0%/day during the proliferative and 3.2%/day during the post-proliferative phase, whereas that of the cytidine nucleotides was more pronounced (9.6%/day and 18.1%/day respectively). The results reveal the existence of partial compartmentalization of pyrimidine ribonucleoside triphosphate pools in the nucleus and cytoplasm of rat liver cells.

Turnover of ribosomal 28S and 18S rRNA during rat liver regeneration.

The turnover of 28S and 18S rRNA was studied in the course of 12 d after partial hepatectomy, including the proliferative (1st to 5th d) and post-proliferative (6th to 12th d) phases of liver regeneration. Turnover data, as the day-to-day rates of synthesis and degradation of 28S and 18S rRNA, were obtained by employing a suitable experimental procedure for the estimation of the increase of the amount of rRNA in the regenerating liver. It was found that 28S and 18S rRNA are accumulated into the cytoplasm and degraded at identical rates both in the proliferative and post-proliferative phases. The turnover of both rRNA moieties is markedly slower during the first 3 d of liver regeneration.

Turnover of ribosomes in regenerating rat liver.

The turnover of ribosomes was studied in regenerating liver and the rates of ribosome degradation in the proliferative (1-5 days) and post-proliferative (6-12 days) phases after partial hepatectomy compared: Ribosome turnover during the first 3 days is slower, the half-life of ribosomes being approximately 300 hr. Ribosome synthesis is markedly activated--39% synthesis rate, which corresponds to 34% accumulation rate per day. The half-life of ribosomes during the second phase is about 107 hr. The rate of synthesis falls down to 19% the net gain of ribosomes being 3% per day.