Imaging β-Cell Function Using a Zinc-Responsive MRI Contrast Agent May Identify First Responder Islets

Abstract

An imaging method for detecting β-cell function in real-time in the rodent pancreas could provide new insights into the biological mechanisms involving loss of β-cell function during development of type 2 diabetes and for testing of new drugs designed to modulate insulin secretion. In this study, we used a zinc-responsive MRI contrast agent and an optimized 2D MRI method to show that glucose stimulated insulin and zinc secretion can be detected as functionally active "hot spots" in the tail of the rat pancreas. A comparison of functional images with histological markers show that insulin and zinc secretion does not occur uniformly among all pancreatic islets but rather that some islets respond rapidly to an increase in glucose while others remain silent. Zinc and insulin secretion was shown to be altered in streptozotocin and exenatide treated rats thereby verifying that this simple MRI technique is responsive to changes in β-cell function.

Keywords: glucose stimulated zinc secretion (GSZS); glucose-stimulated insulin secretion (GSIS); magnetic resonance imaging; metabolic imaging; pancreatic β-cell function; zinc-responsive contrast agent.

Figure 1

(A) Axial T₁-weighted pre- and 5 min post-contrast MRI of rat pancreas after administration of GdL₂ plus saline (top row) or GdL₂ plus glucose (bottom row). The pancreas is outlined with solid cyan in pre-contrast images. The pancreatic tail, left kidney, and major blood vessels (pink arrows) are enhanced post-injection. The cyan arrow represents the splenic vein. [P: pancreas, S: spleen, LK: left kidney, RT: right kidney]. (B) Pancreas signal enhancement over time in rats injected with either GdL₂ plus glucose or GdL₂ plus saline. Rats injected with GdL₂ plus glucose display significantly higher signal enhancement across all time points (p < 0.0001) compared to rats injected with GdL₂ plus saline (p values varied from *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001). The individual data points in (B) are shown as the mean ± SD.

Figure 2

Co-registration experiment demonstrating the correlation of ex vivo MRI “hot spots” with islets as detected by immunohistochemical staining for insulin. (A) MR signals detected in an ex vivo pancreas removed from a rat 5 min after injection of GdL₂ plus glucose. Setting the color scale to “cool” allows visualization of individual hot spots as indicated by the numbered arrows. (B) Islets identified by immunofluorescence staining for insulin, matched to the MR imaging plane shown in (A) demonstrates a majority of visualized “hot spots” in the MR images correspond to islets. (C) H&E staining also identifies islets and blood vessels. Note that some hot spots observed by MRI actually reflect blood vessels (red dotted circles), respectively. Interestingly, some insulin-stained islets do not appear as hot spots in the MR image shown in (A) (yellow circles).

Figure 3

Percent volume of hot spots relative to total volume of the pancreatic tail as detected in axial and coronal slices. The data at each time point are averages ± SD for n = 3.

Figure 4

(A) Representative MRI images of exenatide and STZ-treated rats. Exenatide (top row) results in sustained pancreas tail enhancement with focal hot spots. In contrast, rats treated with STZ showed significantly reduced signal enhancement in the pancreas tail and no hot spots. (B) Comparison of pancreatic tail signal enhancement (SE) in control rats, STZ-treated rats (Type 1 diabetes model), and exenatide-treated rats. All rats were given the same dose of GdL₂ (0.1 mmol/kg) and glucose (2.75 mmol/kg dextrose) prior to imaging. STZ results in significant reduction in SE compared to control and exenatide groups. The individual data points in (B) are shown as the mean ± SD.

Figure 5

(A) MR signal intensities of individual pancreatic hot spots in control rats versus (B) exenatide treated rats over time. These data are consistent with sustained insulin secretion in animals treated with exenatide.