canagliflozin was sufficient to inhibit SGLT2 but not SGLT1 systemically
(19). A previous study has shown that suppressed
glucose excursion is at least partly mediated by enhanced GLP-1
secretion during OGTT in SGLT1/ mice (30). Similar to this
SGLT1 knockout model, the combination treatment, compared
with the teneligliptin alone treatment, markedly augmented
plasma aGLP-1 concentrations along with a much improved
glucose tolerance in ZDF rats in the present study. Thus, it is likely
that intestinal SGLT1 inhibition contributes to improve glucose
tolerance by canagliflozin when combined with teneligliptin.
Systemic SGLT2 inhibition does not affect GLP-1 secretion
because there are no changes in plasma GLP-1 levels in SGLT2-
deficient mice after glucose-loading (28). Thus, although canagliflozin
is an SGLT2 inhibitor, SGLT2 inhibition would not
contribute to the elevation in plasma aGLP-1 levels in ZDF rats. It
is likely that orally administered canagliflozin delays the absorption
of glucose through inhibition of SGLT1 in the upper part
of the small intestine and that GLP-1 is secreted because of
increased glucose levels in the distal part of the small intestine.
While the glucose excursion in the combination group was
greatly suppressed compared with that in the teneligliptin group in
the OGTT, the increase of insulin levels was similar between these
groups. Canagliflozin decreases blood glucose levels by enhancing
excretion of glucose from the kidney independent of insulin action
in hyperglycemic conditions. The similar insulin release with a
small increment of glucose level shows that the glucose-induced
insulin release mechanism was greatly enhanced with the addition
of canagliflozin in the teneligliptin-treated hyperglycemic rats.
In contrast to the insulin level, the plasma aGLP-1 level was greatly
elevated in the rats with combination treatment group compared
with those in the teneligliptin treatment group. Because aGLP-1
enhances glucose-induced insulin release, it is suggested that the
plasma aGLP-1 elevation by combined treatment facilitated
glucose-induced insulin release in the OGTT.
Glucose excursion in the OGTT was greatly reduced with combination
treatment compared with the canagliflozin treatment
alone. GLP-1 is an incretin hormone that enhances postprandial
insulin secretion. Both aGLP-1 and insulin concentrations were
greatly enhanced by the combination treatment compared with the
canagliflozin treatment. It is reasonable to assume that increased
plasma insulin levels further reduces glucose excursion during
OGTT with combined treatment. Therefore, elevation of aGLP-1