Temperature effects on morphological integrity and Ca2+ signaling in freshly isolated murine feed artery endothelial cell tubes

To study Ca2+ signaling in the endothelium of murine feed arteries, we determined the in vitro stability of endothelial cell (EC) tubes freshly isolated from abdominal muscle feed arteries of male and female C57BL/6 mice (5–9 mo, 25–35 g). We tested the hypothesis that intracellular Ca2+ concentration ([Ca2+]i) responses to muscarinic receptor activation would increase with temperature. Intact EC tubes (length: 1–2 mm, width: 65–80 μm) were isolated using gentle enzymatic digestion with trituration to remove smooth muscle cells. A freshly isolated EC tube was secured in a chamber and superfused at 24 (room temperature), 32, or 37°C. Using fura-2 dye, [Ca2+]i was monitored (ratio of fluorescence at 340- to 380-nm wavelength) at rest and in response to bolus doses of ACh (20 nmol to 200 μmol). The morphological integrity of EC tubes was preserved at 24 and 32°C. Based on the Ca2+ Kd values we determined for fura-2 (174 nM at 24°C and 146 nM at 32°C), resting [Ca2+]i remained stable for 180 min at both 24 and 32°C (27 ± 4 and 34 ± 2 nM, respectively), with peak responses to ACh (20 μmol) increasing from ∼220 nM at 24°C to ∼500 nM at 32°C ( P < 0.05). There was no difference in responses to ACh between EC tubes from male versus female mice. When EC tubes were maintained at 37°C (typical in vivo temperature), resting [Ca2+]i increased by ∼30% within 15 min, and gaps formed between individual ECs as they retracted and extruded dye, precluding further study. We conclude that EC tubes enable Ca2+ signaling to be evaluated in the freshly isolated endothelium of murine feed arteries. While Ca2+ responses are enhanced by approximately twofold at 32 versus 24°C, the instability of EC tubes at 37°C precludes their study at typical body temperature.