1450 K. Dube et al. Journal of Insect Physiology 46 2000 1449–1460
1. What are the relative contributions of sequestration of calcium within the tubule, and secretion of calcium
in soluble form into the tubule lumen? Importantly, we have examined both processes in tubules of a single
species and the same stage of development, whereas pre- vious studies have examined either Ca
2 +
secretion by tubules of adult D. melanogaster or calcium seques-
tration by larval tubules of D. hydei. Ca
2 +
transport has been measured across all segments of both anterior and
posterior MTs of adult D. melanogaster. Both the flux of calcium into the tubule cells across the basolateral
membrane basolateral Ca
2 +
flux and the flux across the epithelium and into the lumen transepithelial Ca
2 +
flux have been determined. A reduction in the value of trans-
epithelial relative to basolateral Ca
2 +
flux indicates that Ca
2 +
is retained or sequestered in some way within the tubule cells.
2. What are the mechanisms of MT Ca
2 +
transport? Characterization of Ca
2 +
transport involved measurement of basolateral and transepithelial Ca
2 +
fluxes in response to changes in bathing saline ionic composition or the
addition of pharmacological reagents such as putative inhibitors of calcium-transporting ATPases and cal-
cium channels.
2. Methods and materials
2.1. Insects and fluid secretion assays Adult female flies were selected from a laboratory col-
ony maintained on standard fly medium containing inac- tivated yeast, sucrose and agar supplemented with fresh
active yeast Busto et al., 1999. Tegosept in ethanol and propionic acid were used to prevent mould growth. MTs
were isolated and secreted fluid was analysed as described previously O’Donnell and Maddrell, 1995.
The four MTs consist of an anterior and posterior pair, and each pair is connected to the hindgut through a short
ureter Fig. 1A. For experiments examining transport across the whole MT, a preparation described in O’Don-
nell and Maddrell 1995 was used Fig. 1B. All four tubules, still connected to a very short length of the gut,
were dissected and one pair of tubules was placed in a droplet of bathing saline under paraffin oil. One tubule
of the other pair was cut away and discarded, and the remaining tubule was pulled out into the paraffin oil and
wrapped around a short length 1–2 mm of a fine steel minute pin 0.15 mm o.d. stuck into the Sylgard bottom
of the dish. Fluid was thus collected after it had passed through the entire length i.e. all segments of the two
tubules upstream of their common ureter.
Control saline consisted of in mmol l
21
: 135 NaCl, 20 KCl, 2 CaCl
2
, 8.5 MgCl
2
, 10.2 NaHCO
3
, 4.3
Fig. 1. A Schematic diagram showing the segments of anterior and
posterior Malpighian tubules. Stippling of the distal segment indicates the presence of lumenal Ca
2 +
-rich concretions. B Schematic diagram indicating arrangements for collection of fluid secreted by all segments
of a pair of Malpighian tubules.
NaH
2
PO
4
, 15 HEPES, 20 glucose. Saline pH was adjusted to 7.0. A Ca
2 +
-free saline was made by replace- ment of CaCl
2
with NaCl. Previous studies have found that tubules secrete well in Standard Bathing Medium
SBM consisting of a 1:1 mixture of Schneider’s Insect Medium Sigma and control saline. However, SBM
contains 4 mmol l
21
calcium O’Donnell and Maddrell, 1995, so experiments involving changes in bathing
medium Ca
2 +
concentration required use of an alterna- tive. To preserve the same amino acid concentration as
in the SBM, an amino acid replete saline AARS was used. This contained amino acids at half their concen-
trations found in Schneider’s Insect Medium. AARS was made
by adding
the following
amino acids
1451 K. Dube et al. Journal of Insect Physiology 46 2000 1449–1460
concentrations in mmol l
21
to control saline: 1.7 gly- cine, 7 l-proline, 6.15 l-glutamine, 0.95 l-histidine, 0.55
l-leucine, 4.5 l-lysine, and 1.3 l-valine. The pH of AARS was adjusted to 7.0 and calcium concentrations
were varied by substitution with NaCl to maintain con- stant osmolality. Measurements with Ca
2 +
-selective microelectrodes indicated that nominally Ca
2 +
-free AARS contained 0.02 mmol l
21
Ca
2 +
. Secreted fluid pH and Ca
2 +
concentration were meas- ured with H
+
-selective and Ca
2 +
-selective microelec- trodes based on the ionophores tridodecylamine and
ETH 1001 Fluka Chemical Corp, Ronkonkoma, NY, respectively. Procedures for electrode fabrication have
been described previously Maddrell et al., 1993. Briefly, borosilicate glass pipettes were acid-washed,
dried on a hot plate and silanized by exposure to vapours of dimethyldichlorosilane. Silanization renders the glass
surface hydrophobic and facilitates filling with, and retention of, the hydrophobic ionophore cocktail.
The Ca
2 +
concentration or the pH of secreted fluid droplets was measured under paraffin oil by positioning
the ion-selective and reference microelectrodes in the drop and measuring the potential relative to that in drops
of calibration solutions. A trial-and-error approach was used to determine the optimal extent of silanization.
Insufficient silanization resulted in displacement of the cocktail by aqueous solutions, whereas excess silaniz-
ation resulted in displacement of the cocktail by paraffin oil. Electrodes were calibrated in droplets of calibration
solutions under paraffin oil before and after measure- ment of 3–6 secreted fluid droplets. Calibration solutions
for pH microelectrodes were made from control saline adjusted to differ by 1 pH unit and bracketing the range
of interest. Slopes of pH microelectrodes were 56–59 mV per pH unit. Selectivity of these electrodes for H
+
relative to Na
+
, K
+
and Ca
2 +
were 10
10.4
, 10
9.8
and .10
11.1
, respectively Ammann, 1986. Preliminary measurements indicated that pH of secreted fluid slowly
.1 h became alkaline, presumably due to loss of CO
2
into the paraffin oil. The pH of secreted droplets from isolated MTs was therefore measured as soon as possible
,30 min after collection. Also, pH was measured alter- nately between sample groups so that there was no dif-
ference in the average time between collection and analysis of samples from control and experimental
groups.
Calibration solutions for Ca
2 +
microelectrodes were made from control saline containing 0.2 or 2 mmol l
21
Ca
2 +
. Slopes of Ca
2 +
microelectrodes were 26–28 mV10-fold change in calcium activity. Selectivity of
these electrodes for Ca
2 +
relative to Na
+
, K
+
and Mg
2 +
was 10
5.5
, 10
5.4
and .10
4.9
, respectively Ammann, 1986.
2.2. Drug preparation Drugs were obtained from Sigma and were added
directly to the medium bathing isolated MTs at times indicated for each set of experiments. Diltiazem, verapa-
mil and cAMP were dissolved directly in the bathing medium. Stock solutions of ruthenium red were prepared
in 0.01 mol l
21
NaOH. Stock solutions of thapsigargin, A23187 and nifedipine were prepared in ethanol so that
the final concentration of ethanol in saline bathing the tubules was ,1. Previous studies have shown that
tubules are unaffected by 1 ethanol O’Donnell et al., 1996. Salines for control and experimental tubules con-
tained the same concentration of ethanol.
2.3. Basolateral Ca
2 +
flux measurements Pairs of MTs were dissected under saline and placed
under liquid paraffin oil into 10 µ
l droplets of bathing medium containing
45
Ca
2 +
. Preliminary measurements showed that isolated MTs accumulated
45
Ca
2 +
linearly over periods of 10–240 min, so exposure times 40 min
were used in all subsequent experiments. MT pairs were removed and washed in “cold” Ca-free saline containing
2 mmol l
21
EGTA. The binding affinity of EGTA is much higher than that of biological ligands, thus ensur-
ing removal of
45
Ca
2 +
bound non-specifically to the tubule surfaces. Analysis of
45
Ca
2 +
levels in the wash droplets indicated that three washes of 5 s each were
sufficient to remove .92 of surface-bound calcium. MTs were then lysed osmotically by placement in 10
µ l
of deionized H
2
0 under paraffin oil. Both the water and the MTs were then transferred by pipette to 4 ml of
Beckman Ready Safe Liquid Scintillation Cocktail.
45
Ca
2 +
content was determined by counting for 10 min in a LKB Wallac 1217 liquid scintillation counter. Initial
and final counts of the bathing medium were not signifi- cantly .5 different, and the initial value was used
to determine the specific activity of the bathing saline. After subtraction of background counts, calcium flux
pmol min
21
tubule
21
was calculated from the meas- ured cpm, using the specific activity cpm mol
21
of the saline containing
45
Ca
2 +
and the duration of exposure. Background values were determined by measuring blank
vials of 4 ml liquid scintillation cocktail and 1 µ
l of “cold” saline.
2.4. Transepithelial Ca
2 +
flux measurements Methods for fly dissection and collection of fluid
secreted by MTs isolated under paraffin oil are described in Dow et al. 1994. Droplets of secreted fluid were
collected and their volumes were calculated as p6d
3
, where d is the diameter of the spherical droplet measured
by an ocular micrometer. Secretion rates nl min
21
were determined by dividing the volume nl of the droplet
1452 K. Dube et al. Journal of Insect Physiology 46 2000 1449–1460
by the interval min over which the droplet formed. Cal- cium concentration of secreted fluid droplets was meas-
ured by a Ca
2 +
-selective microelectrode, as described previously O’Donnell and Maddrell, 1995. Transepi-
thelial Ca
2 +
flux pmol min
21
tubule
21
was calculated as the product of fluid secretion rate nl min
21
tubule
21
and secreted fluid Ca
2 +
concentration mmol l
21
2.5. Statistics Where appropriate, data are presented as means
± SEM.
Statistical significance of differences between means was determined using Student’s t-test two-tailed,
ANOVA or Tukey Test, taking p,0.05 as the critical level.
3. Results
