Intrathecal Morphine and Effect on Opioid Consumption and Functional Recovery after Pancreaticoduodenectomy

BACKGROUND

Single-shot intrathecal morphine (ITM) is an effective strategy for postoperative analgesia, but there are limited data on its safety, efficacy, and relationship with functional recovery among patients undergoing pancreaticoduodenectomy.

STUDY DESIGN

This was a retrospective review of patients undergoing pancreaticoduodenectomy from 2014 to 2020 as identified by the institutional NSQIP Hepato-pancreato-biliary database. Patients were categorized by having received no spinal analgesia, ITM, or ITM with transversus abdominus plane block (ITM+TAP). The primary outcomes were average daily pain scores from postoperative days (POD) 0 to 3, total morphine equivalents (MEQ) consumed over POD 0 to 3, and average daily inpatient MEQ from POD 4 to discharge. Secondary outcomes included the incidence of opioid related complications, length of stay, and functional recovery.

RESULTS

A total of 233 patients with a median age of 67 years were included. Of these, 36.5% received no spinal analgesia, 49.3% received ITM, and 14.2% received ITM+TAP. Average pain scores in POD 0 to 3 were similar by mode of spinal analgesia (none [2.8], ITM [2.6], ITM+TAP [2.3]). Total MEQ consumed from POD 0 to 3 were lower for patients who received ITM (121 mg) and ITM+TAP (132 mg), compared with no spinal analgesia (232 mg) (p < 0.0001). Average daily MEQ consumption from POD 4 to discharge was lower for ITM (18 mg) and ITM+TAP (13.1 mg) cohorts compared with no spinal analgesia (32.9 mg) (p = 0.0016). Days to functional recovery and length of stay were significantly reduced for ITM and ITM+TAP compared with no spinal analgesia. These findings remained consistent through multivariate analysis, and there were no differences in opioid-related complications among cohorts.

CONCLUSIONS

ITM was associated with reduced early postoperative and total inpatient opioid utilization, days to functional recovery, and length of stay among patients undergoing pancreaticoduodenectomy. ITM is a safe and effective form of perioperative analgesia that may benefit patients undergoing pancreaticoduodenectomy.

Early Prediction of Length of Stay After Pancreaticoduodenectomy

BACKGROUND

Enhanced recovery after surgery (ERAS) is an evidence-based clinical pathway designed to standardize and optimize care. We studied the impact of ERAS and sought to identify the most important recommendations to predict shorter length of stay (LOS) after pancreaticoduodenectomy (PD).

METHODS

We retrospectively reviewed all patients undergoing PD at our institution between January 2014 and June 2018. We compared clinicopathologic outcomes for patients before and after ERAS implementation. We defined "A-recommendations" as those that were graded "strong" and had "moderate" or "high" levels of evidence. We then compared outcomes of the ERAS group with adherence to "A-recommendations" and performed a subset analysis of "A-recommendations" over the first 72 h after surgery, which we termed "early factors".

RESULTS

A total of 191 patients underwent PD during the study period. We excluded 87 patients who had minimally invasive PD (22), vascular reconstruction (53), or both (12). Of the 104 patients studied, 56 (54%) were pre-ERAS and 48 (46%) were ERAS. There were no differences in comorbidities or demographics between these groups, and morbidity, mortality, and readmission rates were also similar (P > 0.6). Median LOS was 3.5 d shorter in the ERAS group (7 versus 10.5 d, P < 0.001). Adherence to "A-recommendations" within ERAS was associated with a decreased LOS (r = -0.52 P = 0.0001). Patients with >5 "early factors" had a median LOS of 6 d, whereas patients with <5 "early factors" had a median LOS of 9 d (P = 0.008).

CONCLUSIONS

ERAS is an effective protocol that standardizes care and reduces LOS after PD. Implementation of ERAS resulted in a 3.5-day reduction in our LOS with no change in morbidity, mortality, or readmissions. Adherence to ERAS protocol "A-recommendations" and ≥5 "early factors" may be predictive of shortened LOS.

Antibody blockade of semaphorin 4D to sensitize pancreatic cancer to immune checkpoint blockade

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Background: Pancreatic ductal adenocarcinoma (PDAC) carries a dismal prognosis, and will soon become the second leading cause of cancer mortality. Unfortunately, T-cell directed immunotherapies have failed to demonstrate efficacy in PDAC. These failures may in part be mediated by an immunosuppressive tumor microenvironment (TME). Semaphorin 4D (Sema4D) is a glycoprotein which binds its cognate receptors Plexin B1/B2. Here we present our work in blocking Sema4D in a murine model of PDAC. Methods: C57b/6 mice were orthotopically injected with PDAC line (KP2) derived from KRASG12D,TP53Flox/Wt;P48-Cre autochthonous tumors and confirmed for disease by ultrasound. Mice were treated with FOLFIRINOX (5-FU, Irinotecan, Oxaliplatin, weekly), immune checkpoint blockade (ICB) (anti-PD1, anti-CTLA-4 mAbs bi-weekly), and anti-Sema4D mAB (bi-weekly). Human and mouse circulating and tumor infiltrating leukocytes were interrogated through flow cytometry (FACS) for immune subset and expression of Sema4D and Plexin receptors. Archived human PDAC tissues were assessed through quantitative immunohistochemistry (IHC) for presence of Sema4D positive infiltrate. Results: Both FACS and IHC analysis of human PDAC specimens confirm the presence and increased prevalence over normal pancreata of Sema4D lymphocytes and Plexin B1/B2 expressing tumor associated macrophages (TAMs). KP2 orthotopically injected mice exhibited longer survival when treated with the triple combination of FOLFIRINOX, ICB, and anti-Sema4D antibody, compared to FOLFIRINOX alone, FOLFIRINOX plus ICB, or FOLFIRINOX plus anti-Sema4D antibody (P < 0.02). Flow cytometric analysis of anti-Sema4D and ICB treated murine tumors show a doubling of penetration by CD 8+ effector T cells within tumors compared to control groups (P = 0.03). A loss in Sema4D fluorescence signal via FACS in tumor-infiltrating CD3+ leukocytes was observed in mice treated with anti-SEMA4D, confirming penetration and target blockade within the TME. Conclusions: Sema4D and Plexin B1/B2 leukocytes penetrate human PDAC tumors, and treatment with Sema4D blocking antibody improved response to ICB in combination with standard of care FOLFIRINOX in preclinical murine studies.

Use of image-guided bone scalpel for resection of spine tumors: technical note

In the literature, the use of navigation for spine tumor surgery has largely centered on implant placement. We describe the cases of two patients with spinal tumors on whom we utilized our resection technique of registering an ultrasonic bone scalpel (UBS) to a navigation system. In both cases, we achieved a satisfactory tumor resection with negative margins and excellent neurologic outcomes. We feel that using the navigation-registered UBS is a valuable tool to increase the operator's ability to achieve desired resections while minimizing the neurologic deficits and operative morbidity associated with these challenging surgical cases.

Long-Term Survival of a Patient with Metastatic Small-Cell Carcinoma of the Stomach Treated with Radiation Therapy

Small-cell carcinoma (SCC), or high-grade neuroendocrine carcinoma of the stomach, is a rare subtype of extra-pulmonary SCC which is almost invariably lethal. Gastric SCC often presents with local symptoms indistinguishable from other primary stomach cancers; however, both regional and distant spread are common at the initial presentation. Depending on symptoms and patient performance status, treatment typically consists of chemotherapy or resection followed by adjuvant chemotherapy, as even patients with limited stage gastric SCC likely have micrometastatic disease at the time of diagnosis. In this case report, we describe the long-term survival of a 75-year-old male with recurrent oligometastatic high-grade neuroendocrine carcinoma of the stomach treated with radiation therapy (RT) alone. He presented with abdominal pain and dyspepsia and was found to have a 6 cm locally invasive node-positive gastric SCC initially treated with extensive surgical resection. He was not a candidate for adjuvant chemotherapy, and surveillance imaging subsequently confirmed metachronous liver and local recurrences within 1 year after surgery, which were managed with stereotactic body RT and conventional radiation, respectively. An additional para-aortic nodal recurrence was treated with intensity-modulated radiotherapy 7 years after surgery with good response. He tolerated all RT courses without notable radiation-related toxicity and remains in complete remission 11 years after initial diagnosis.

The influence of staple size on fistula formation following distal pancreatectomy

BACKGROUND

Pancreatic fistula continues to be a source of significant morbidity following distal pancreatic resections. The technique of pancreatic division varies widely among surgeons, and there is no evidence that identifies a single method as superior. In our practice, the technique of distal pancreatic resection has evolved from cut-and-sew to stapled technique with green and recently white cartridge. The aim of our study was to evaluate the rate of clinically significant fistulas [International Study Group on Pancreatic Fistula (ISGPF) grade B or C] following distal pancreatectomy and to identify variables associated with a low rate of fistula development.

METHODS

Clinical records of all patients who underwent distal pancreatic resections between February 1999 and July 2010 by a single surgeon were retrospectively reviewed focusing on the incidence and type of pancreatic fistula as defined by ISGPF. Study variables included age, gender, surgical approach, extent of resection, ASA classification, type of stapler cartridge, use of Seamguard™, and ISGPF classification. Statistical analysis was performed using Fisher's exact test, and univariate and multivariate logistic regression.

RESULTS

Sixty-four patients (median age 60, range 21-85; 54% male) underwent distal pancreatic resection (laparoscopy 50% vs. open 50%). The most common indications were pancreatic adenocarcinoma (N = 15; 23%) and neuroendocrine neoplasms (N = 14; 22%). Clinically significant pancreatic fistula developed in 24% (N = 15). The rate of fistula with cut-and-sew technique was 36% (4/11), with stapled green cartridge 31% (9/29) and only 5% (1/21) with stapled vascular cartridge. Univariate logistic regression identified vascular cartridge size (p = 0.04, OR 0.11) and open stapled technique (p = 0.05, OR 0.12) as variables significantly associated with a low fistula rate. Both vascular cartridge size (p = 0.05, OR 0.10) and open stapled technique (p = 0.04, OR 0.08) remained significant when analyzed by multivariate logistic regression. Division of pancreatic parenchyma with vascular cartridges resulted in significantly (p = 0.03, OR 9.0) lower fistula rate compared to green cartridges. The use of Seamguard™ did not affect fistula rate (16% vs. 27%; p = 0.34) nor did the performance of multivisceral resection vs. distal pancreatectomy/splenectomy alone (21% vs. 23%, p = 1.0).

CONCLUSION

The optimal technique of pancreatic division has not been conclusively established. Dividing the pancreas utilizing vascular (2.5 mm) staple cartridges significantly decreased the rate of clinically significant pancreatic fistula and we have changed our practice accordingly. A prospective randomized trial is necessary to validate these results.

Solitary fibrous tumor of the pancreas: a case report

Solitary fibrous tumor (SFT) is an unusual mesenchymal neoplasm that most often arises in the pleura; however, it has recently been described in a number of extrapleural sites. This report describes an extremely rare case of a benign SFT arising in the pancreas. A 41-year-old woman presented in the clinic with right upper abdominal pain. Subsequent ultrasonographic studies revealed a 1.5x1.5x1.4 cm hypoechoic mass within the pancreatic body, which was later confirmed on both helical computerized tomography and magnetic resonance imaging studies. An endocrine tumor was clinically suspected. Laparoscopic enucleation of the mass was performed. Microscopically, the tumor was composed of bland uniform spindle cells arranged between collagen bundles. On immunohistochemical studies, these spindle cells were positive for CD34 and bcl-2 but negative for cytokeratin (AE1+AE3 and Cam5.2), smooth muscle actin, desmin, S-100, and c-kit. Based on the light microscopic morphology and immunohistochemical staining profile, the diagnosis of SFT was rendered.

Pancreaticoduodenectomy in the presence of superior mesenteric venous obstruction

The study goal was to determine the technical feasibility and outcomes associated with pancreaticoduodenectomy for periampullary malignancies with near (>80%) or complete (100%) superior mesenteric venous (SMV) obstruction. A retrospective examination of 11 patients with high-grade or complete SMV obstruction who underwent pancreaticoduodenectomy at five academic medical centers is reviewed. Pancreaticoduodenectomy for locally advanced periampullary malignancies causing high-grade or complete SMV obstruction is technically feasible. Operative approaches and outcomes are presented. One 30-day death was observed. Median survival of the cohort is 18 months. Survivals exceeding 2 years post-resection have been observed. In a number of cases, significant palliation of pain and of biliary and duodenal obstruction were achieved. Based on this initial series, pancreaticoduodenectomy in the presence of near or total SMV obstruction is feasible, may result in an R0 resection, and may be beneficial in select patients with a periampullary malignancy. We suggest such an approach be considered particularly following completion of neoadjuvant therapy without systemic progression. Further studies and more long-term follow-up at high-volume centers are required, however, to better determine the indications and potential benefit of such an undertaking.

Laparoscopic spleen-preserving distal pancreatectomy

Distal pancreatectomy with spleen preservation may be the preferred procedure for certain benign tumors and cystic lesions of the pancreatic body or tail. Alternatively, laparoscopic removal including either distal pancreatectomy with splenectomy or splenic-preservation with ligation of the splenic vessels have also been described. We describe, herein, our method to perform spleen-preserving laparoscopic distal pancreatectomy that preserves the splenic vessels and hence splenic function. The described technique of spleen-preserving distal pancreatectomy has been used in two patients with favorable results. Both patients underwent laparoscopic distal pancreatectomy with splenic conservation for an oligocystic serous cystadenoma and serous cystadenoma. Operative time was 3-6 hours with total blood loss of less than 200 cc in both cases. The length of stay in the hospital was 4-8 days and both patients returned to work within 3 weeks. Laparoscopic spleen-preserving distal pancreatectomy should be considered for younger patients with select body or tail lesions that are not candidates for less extensive procedures.

Small bowel obstruction due to a paracolonic retroperitoneal hernia

Hernias involving the retroperitoneum are unusual. The most common of these are the paraduodenal hernias. A retroperitoneal hernia occurring from a lateral defect in the colonic retroperitoneal attachments is presented. We believe that this case represents a newly recognized variant of retroperitoneal hernias.

Small Bowel Obstruction Due to a Paracolonic Retroperitoneal Hernia

Hernias involving the retroperitoneum are unusual. The most common of these are the paraduodenal hernias. A retroperitoneal hernia occurring from a lateral defect in the colonic retroperitoneal attachments is presented. We believe that this case represents a newly recognized variant of retroperitoneal hernias.

Dynamic retention: a technique for closure of the complex abdomen in critically ill patients

Management of the open abdomen in the setting of massive visceral swelling or extensive intra-abdominal abscess may pose an extremely difficult surgical scenario. We herein describe the technique and results of dynamic-retention sutures used in 13 patients with abdominal catastrophes after trauma, vascular reconstruction, tumor extirpation, and intra-abdominal infection. Three of these patients died during their acute care hospitalization. The remaining 10 patients were discharged to home with no resultant fistulas and 1 recurrent hernia (10%). Dynamic-retention sutures provide a useful technique for the closure of the complex surgical abdomen. We observed a low complication rate. In properly selected patients, this technique avoids the use of mesh or additional surgical procedures such as skin grafting or plastic surgical reconstruction of the abdominal wall.

G207, modified herpes simplex virus type 1, kills human pancreatic cancer cells in vitro

Pancreatic cancer is often fatal, and further effective therapeutic options are needed. This study was designed to assess whether the replication-restricted herpes simplex virus, G207, was effective in killing human pancreatic cancer cells in vitro. G207, a multimutated strain of herpes simplex virus type 1 carrying lacZ reporter gene, is capable of efficient cytolytic growth in many dividing cells, including certain tumor cells, but not in nondividing cells. Three human pancreatic cell lines, AsPC-1, MIA PaCa-2, and BxPC-3, were infected with G207 at different multiplicities of infection. After 24 hours, expression of the lacZ reporter gene was tested using a histochemical X-gal assay. In addition, cell lines were infected with G207 for 24 to 48 hours; then the virus obtained from cell pellets and media supernatant was used to infect Vero cells to obtain G207 titers by plaque assay. To assess whether increasing viral immediate early gene expression would improve cytolysis and virus production, similar experiments were performed with the addition of 0.5 mmol/L of hexamethylene bisacetamide (HMBA) 1 hour after viral infection. Finally, MTS cell viability assays were performed to measure viable cells at 24 to 96 hours post infection. The X-gal assay data revealed a viral dose-dependent b-galactosidase expression, indicating G207 infectivity and expression of the lacZ reporter gene. Plaque assays demonstrated a viral dose-dependent increase in plaque formation, indicating viral production from all three cell lines. In addition, HMBA data indicated a modest increase in viral production. The MTS assay data indicated a dose-dependent cytotoxicity for G207 in the cell lines tested. G207 infects, replicates in, and is cytotoxic to the above-listed human pancreatic cell lines in vitro and warrants therapeutic evaluation in models of pancreatic cancer.

A newly recognized entity: intraductal "oncocytic" papillary neoplasm of the pancreas

Intraductal papillary-mucinous tumors of the pancreas are increasingly recognized, and their characteristic endoscopic and radiological features are well reported in the literature in recent years. Oncocytic features in these tumors are uncommon and unrecognized. Intraductal oncocytic papillary neoplasm is a distinct pancreatic tumor and is a recently recognized entity. We report a case of a 69-yr-old patient who presented with symptoms mimicking pancreatitis, resulting in delay in the diagnosis of her pancreatic tumor. She underwent a successful Whipple's procedure and subsequently has remained well. The resected specimen showed an intraductal oncocytic papillary-mucinous neoplasm. The entity is new and the literature information is inadequate at present to judge the biological behavior of this tumor. We discuss this recently recognized entity.

Myocardial heat shock gene expression in pigs is dependent on superoxide anion generated at reperfusion

The heat shock response is a conserved response to cell injury. We sought to determine if ischemia alone versus events at reperfusion stimulated expression of the major heat shock protein (hsp-72) in a clinically relevant model of global myocardial ischemia in pigs. Pigs were placed on nonpulsatile cardiopulmonary bypass. Serial transmural cardiac biopsies were taken at baseline following 20 min of normothermic global ischemia (induced by crossclamping the aorta) and at 20, 40, and 60 min of reperfusion. Test animals received a bolus and subsequent aortic root infusion of superoxide dismutase (total 7,500 U/kg) beginning just prior to reperfusion. Hsp-72 mRNA abundance was estimated from Northern blots. We found that hsp-72 mRNA was not induced following 20 min of ischemia but accumulated to high levels within 20 min of reperfusion. Intravascular administration of superoxide dismutase at reperfusion eliminated hsp-72 mRNA induction. We conclude that in the postischemic myocardium, hsp-72 gene expression is dependent on superoxide anion generation at reperfusion. In this setting, hsp-72 gene expression may reflect a specific response to oxidative injury rather than a more general response to metabolic stress associated with ischemia.

Induction of heat-shock gene expression in postischemic pig liver depends on superoxide generation

BACKGROUND/AIMS

Both hemorrhagic and cardiogenic shock are associated with hepatic shock gene expression at resuscitation. This study investigated the potential role of intravascular superoxide anion as a proximal trigger of heat shock protein gene expression.

METHODS

Preanesthetized pigs were subjected to 120 m of total warm hepatic ischemia. The survival model consisted of warm, total hepatic ischemia and reperfusion (with active portal-systemic bypass) followed by reperfusion and survival for 3 days. Serial hepatic biopsy samples were evaluated for the expression of heat shock protein 72 (HSP-72) messenger RNA (mRNA) by Northern and Western analysis and by in situ RNA hybridization. The possible role of intravascular O2- as a mediator of heat shock response was evaluated by its specific inhibition by the intravenous infusion of recombinant human superoxide dismutase (SOD).

RESULTS

Ischemia for 120 minutes followed by 60 minutes of reperfusion caused accumulation of HSP-72 mRNA. Transcripts were localized to hepatocytes. HSP-72 mRNA was detected neither following ischemia alone nor when SOD was infused for 15 minutes at reperfusion. Three days later, transcripts were not detectable, but HSP-72 protein accumulated irrespective of SOD administration.

CONCLUSIONS

Warm hepatic ischemia induces the hepatocyte expression of HSP-72 at reperfusion by a mechanism that is dependent upon the superoxide anion, probably generated intravascularly. However, the transient dismutation of superoxide is insufficient to suppress subsequent accumulation of HSP-72.

Regulation of acute phase gene expression following surgery and endotoxin administration in the anesthetized pig

BACKGROUND

The hepatic acute phase response (APR) reflects an organism's integrated response to stress. This APR results in augmented synthesis and secretion of specific procoagulants and antiproteases and a complementary decrease in the synthesis and secretion of several constitutive proteins, such as albumin. The cytokines tumor necrosis factor (TNF) or interleukin-6 (IL-6) have been identified as proximal mediators of the APR in response to endotoxin stress. The authors hypothesized that TNF, IL-6, or both would be the proximal mediators of the APR in response to anesthesia and surgical stress.

METHODS

The effects of a standardized surgical stress on the APR in pigs under general anesthesia with sodium pentobarbital and ketamine hydrochloride was investigated. Acute phase gene transcription was assayed in nuclei from serial liver biopsies obtained before and after 2.5 h of surgical stress, and after endotoxin administration. Tumor necrosis factor and IL-6 mRNA levels in this liver tissue were examined by Northern blot hybridization, and simultaneous plasma levels of these cytokines were measured using bioassays.

RESULTS

The transcription rates of three positive acute phase genes--chymotrypsin inhibitor, inter-alpha-trypsin inhibitor and beta-fibrinogen--increased seven-, six-, and twofold, respectively (P < 0.05), and the transcription rate of albumin, a negative acute phase gene, decreased to 34% of baseline (P < 0.01) during the 2.5 h of anesthesia and surgical stress. During this initial 2.5 h, plasma concentrations of TNF and IL-6 did not change. Hepatic IL-6 mRNA expression was never observed, and TNF mRNA expression was undetectable in six of seven pigs. Subsequent 10-micrograms/kg endotoxin administration caused 20- and 100-fold increases in plasma concentrations of TNF and IL-6, respectively (P < 0.01), and were associated with substantial hepatic expression of the TNF and IL-6 mRNAs. These increments in cytokines were not associated with any further increase in the acute phase gene transcription rates. Thus, the APR was initially regulated at the transcriptional level during surgical stress independent of, and not augmentable by, an endotoxin-provoked increase in either plasma levels or hepatic mRNA expression of TNF or IL-6.

CONCLUSIONS

Surgical stress induced hepatic acute phase gene transcription within 2.5 h in the absence of either systemic or local (hepatic) increases in TNF or IL-6. Subsequent endotoxin-induced increases in TNF or IL-6 did not alter this surgical stress-induced acute phase gene transcription.

Heat-shock gene expression excludes hepatic acute-phase gene expression after resuscitation from hemorrhagic shock

BACKGROUND

To determine whether the gene expression of both acute-phase reactants (APR) and the major heat-shock protein (hsp-72) can occur simultaneously, transcriptional rates were measured during shock and resuscitation.

METHODS

A nuclear runoff technique was applied to hepatic biopsy specimens obtained from pigs before shock, during 40% blood volume hemorrhagic shock (1 and 2 hours), and after resuscitation (4 and 6 hours).

RESULTS

Shock-induced transcription of hsp-72 was elevated elevenfold over sham operation at 2 hours (p less than 0.02, Mann-Whitney rank test). Individually shocked animals did not transcribe both classes of stress genes but segregated into two groups: (1) strong APR transcriptional responders and (2) hsp-72 transcriptional responders. In group 2, APR transcription was significantly suppressed. Antichymotrypsin transcription was an average of eighteenfold lower in group 2 versus group 1 (p less than 0.05 at 1,2, and 6 hours).

CONCLUSIONS

Different classes of stress protein genes are not transcribed simultaneously. We infer that their increased accumulation at the mRNA level is the result of sequential transcription. Hsp-72 transcription excludes that of the APR genes that may be critical to survival after stress.

4.5S RNA is encoded by hundreds of tandemly linked genes, has a short half-life, and is hydrogen bonded in vivo to poly(A)-terminated RNAs in the cytoplasm of cultured mouse cells

4.5S RNA is a group of RNAs 90 to 94 nucleotides long (length polymorphism due to a varying number of UMP residues at the 3' end) that form hydrogen bonds with poly(A)-terminated RNAs isolated from mouse, hamster, or rat cells (W. R. Jelinek and L. Leinwand, Cell 15:205-214, 1978; F. Harada, N. Kato, and H.-O. Hoshino, Nucleic Acids Res. 7:909-917, 1979). We have cloned a gene that encodes the 4.5S RNA. It is repeated 850 (sigma = 54) times per haploid mouse genome and 690 (sigma = 59) times per haploid rat genome. Most, if not all, of the repeats in both species are arrayed in tandem. The repeat unit is 4,245 base pairs long in mouse DNA (the complete base sequence of one repeat unit is presented) and approximately 5,300 base pairs in rat DNA. This accounts for approximately 3 X 10(6) base pairs of genomic DNA in each species, or 0.1% of the genome. Cultured murine erythroleukemia cells contain 13,000 molecules per cell of the 4.5S RNA, which can be labeled to equilibrium in 90 min by [3H]uridine added to the culture medium. The 4.5S RNA, therefore, has a short half-life. The 4.5S RNA can be cross-linked in vivo by 4'-aminomethyl-4,5',8-trimethylpsoralen to murine erythroleukemia cell poly(A)-terminated cytoplasmic RNA contained in ribonucleoprotein particles.