Objective—To compare hepatic metabolism of
pyrrolizidine alkaloids (PAs) between sheep and cattle
and elucidate the protective mechanism of sheep.
Sample Population—Liver microsomes and cytosol
from 8 sheep and 8 cattle.
Procedure—The PA senecionine, senecionine N-oxide
(nontoxic metabolite) and 6,7-dihydro-7-hydroxy-
1-hydroxymethyl-5H-pyrrolizine (DHP; toxic metabolite)
were measured in microsomal incubations. The
kcat (turnover number) was determined for DHP and
N-oxide formation. Chemical and immunochemical
inhibitors were used to assess the role of cytochrome
P450s, flavin-containing monooxygenases (FMOs),
and carboxylesterases in senecionine metabolism.
The CYP3A, CYP2B, and FMO concentrations and
activities were determined, in addition to the role of
glutathione (GSH) in senecionine metabolism.
Results—DHP concentration did not differ between
species. Sheep formed more N-oxide, had higher N-oxide
kcat, and metabolized senecionine faster than
cattle. The P450 concentrations and isoforms had a
large influence on DHP formation, whereas FMOs
had a large influence on N-oxide formation. In cattle,
CYP3A played a larger role in DHP formation than in
sheep. FMO activity was greater in sheep than in cattle.
Addition of GSH to in vitro microsomal incubations
decreased DHP formation; addition of cytosol
decreased N-oxide formation.
Conclusions and Clinical Relevance—Hepatic
metabolism differences alone do not account for the
variation in susceptibility seen between these
species. Rather, increased ruminal metabolism in
sheep appears to be an important protective mechanism,
with hepatic enzymes providing a secondary
means to degrade any PAs that are absorbed from the
rumen. (Am J Vet Res 2004;65:1563–1572)
Objective—To determine whether iontophoretic
administration of dexamethasone to horses results in
detectable concentrations in synovial fluid, plasma,
Animals—6 adult mares.
Procedure—Iontophoresis was used to administer
dexamethasone. Treatments (4 mA for 20 minutes)
were administered to a tarsocrural joint of each mare.
The drug electrode contained 3 ml of dexamethasone
sodium phosphate at a concentration of 4 or 10
mg/ml. Samples of synovial fluid, blood, and urine
were obtained before and 0.5, 4, 8, and 24 hours after
each treatment. All samples were tested for dexamethasone
using an ELISA. Synovial fluid also was evaluated
for dexamethasone, using high-performance
Results—The lower and upper limits of detection for
dexamethasone in synovial fluid with the ELISA were
0.21 and 1.5 ng/ml, respectively. Dexamethasone
administered at a concentration of 10 mg/ml was
detected by the ELISA in synovial fluid of 5 mares
from 0.5 to 24 hours and in urine of 4 mares from 0.5
to 8 hours after each treatment, but it was not detected
in plasma. Mean synovial fluid concentration of
dexamethasone was 1.01 ng/ml. Dexamethasone
administered at a concentration of 4 mg/ml was
detected by the ELISA in urine of 2 mares at 0.5 and
4 hours after treatment, but it was not detected in
synovial fluid or plasma.
Conclusion and Clinical Relevance—Iontophoresis
cannot be considered an effective method for delivery
of dexamethasone to synovial fluid of horses,
because drug concentrations achieved in this study
were less than therapeutic concentrations. (Am J Vet
Procedure—Llamas were allocated to 1 of 3 groups
(3 llamas/group). Fentanyl patches (each providing
transdermal delivery of 75 µg of fentanyl/h) were
placed on shaved areas of the antebrachium of all llamas.
In group 1, llamas were treated with 1 patch
(anticipated fentanyl dosage, 75 µg/h). In group 2, llamas
were treated with 2 patches (anticipated fentanyl
dosage, 150 µg/h). In group 3, llamas were treated
with 4 patches (anticipated fentanyl dosage,
300 µg/h). For each llama, the degree of sedation was
assessed by use of a subjective scoring system and a
blood sample was collected for determination of
serum fentanyl concentration at 12, 24, 36, 48, 60,
and 72 hours after patch placement.
Results—Following the placement of 4 patches,
mean ± SD serum fentanyl concentration in group 3
llamas reached 0.3 ± 0.08 ng/mL within 12 hours. This
concentration was sustained for 72 hours. In group 2,
application of 2 patches provided inconsistent results;
in group 1, application of 1 patch rarely provided measurable
serum fentanyl concentrations. No llamas
became sedated at any time.
Conclusions and Clinical Relevance—Results suggest
that application of four 75 µg/h fentanyl patches
provides consistent, sustained serum fentanyl concentrations
without sedation in llamas. However, the
serum concentration of fentanyl that provides analgesia
in llamas is not known. (Am J Vet Res