CHRONOMODULATED NIFEDIPINE ADMINISTRATION AND MICROVASCULAR COMPLICATIONS IN GLIMEPIRIDE-TREATED HYPERGLYCAEMIC RATS

ABSTRACT

Diabetic complications pose increasing public health problems worldwide. Glimepiride, a sulphonylurea is among the most frequently used anti-diabetic drugs, and is often combined with other drugs for the management of type 2 diabetes mellitus. Nifedipine, a calcium channel blocker is recommended for its role in mitigating microvascular complications especially in delaying the onset of diabetic nephropathy. Interaction between both drugs leads to regimen failure when simultaneously administered. However, established diurnal variation in nifedipine pharmacokinetics can ameliorate the benefits of nifedipine-glimepiride co-administration. This study investigates the effect of 21-day chronomodulated nifedipine administration on glycaemic control and microvascular complications in glimepiride-treated streptozotocin-induced hyperglycaemic rats. All animals were treated daily per oral. Fasting and random blood glucose were assessed on days 0, 3, 7, 14, 21, and days 0, 6, 10, 13 20 respectively. Peripheral neuropathy was evaluated using the paw pressure and tail immersion tests on days 7, 14 and 21. At the end of drug treatment, nephropathy and retinopathy were evaluated by determining levels of some serum renal and ocular markers; in addition to histological assessment of the kidney and retina. Administration of glimepiride alone at 8 pm significantly (p<0.01) reduced blood glucose levels on days 7, 14 and 21 when compared to initial values but had no significant effect on outcome of microvascular complications. Concurrent administration of glimepiride and nifedipine at 8 pm impaired glycaemic control and exacerbated microvascular complications. In contrast, treatment with glimepiride at 8 pm and nifedipine at 8 am significantly (p<0.01) improved glycaemic control in a manner similar to the group treated with glimepiride alone at 8 pm. The paw pressure and tail immersion tests showed significant (p<0.05) increase in paw withdrawal latency and mean reaction time respectively on days 14 and 21 in comparison to values on day 7, indicating amelioration in peripheral neuropathy. The serum protein and albumin levels were significantly (p<0.05) higher in this group when compared with diabetic controls. The serum urea levels and relative kidney weights were also significantly (p<0.05) lower when compared to the diabetic control. These indicate improvement in the prognosis of nephropathy. Similarly, the serum triglyceride and cholesterol levels were significantly (p<0.05) lower when compared to the diabetic control. Also, the serum magnesium level was significantly (p<0.05) higher when compared to diabetic control. This indicates protection against retinopathy and is supported by histological findings where photomicrograph of the retina shows normal features similar to non-diabetic control. The findings from this study suggest that diurnal variation in pharmacokinetics can be effectively used to advantage, resulting in glimepiride and nifedipine being successfully co-administered without losing the glucose lowering benefits of glimepiride, while simultaneously delaying the progression and improving prognosis of microvascular complications.

CHAPTER ONE

1.0 Introduction

1.1 Background

Circadian rhythm is any biological process that displays an endogenous (“built in”, self sustained) and entrainable (can be adjusted or reset) oscillation of about twenty-four hours (Halberg et al., 2003). These rhythms are driven by a circadian clock and have been widely observed in plants, animals, fungi and cyanobacteria. The term circadian comes from the Latin word “circa” meaning “around” (or “approximately”) and “diem” or “dies” meaning “day” (Carskadon et al., 1999).

The body’s primary circadian clock resides deep in the hypothalamus, in the suprachiasmatic nuclei (SCN), though other local clocks are also found in tissues and organs throughout the body, including the pancreas, lungs, liver, heart and skeletal muscles (Reppert and Weaver, 2002). These clocks are in synchrony with the master clock and operate on a 24 hour circadian cycle that governs functions such as sleeping and waking (Akerstedt and Gillberg, 1981), rest and activity, fluid balance, body temperature (Redfern et al., 1991), cardiac output, oxygen consumption, metabolism (Waterhouse et al., 1990) and endocrine gland secretion (Redfern et al., 1991).

Many functions within the human body vary considerably within a day and as such, a number of disease conditions follow circadian variations (Devdhawala and Seth 2010). Cardiovascular diseases (Hermida et al., 2011), diabetes mellitus (Friedman and Banerji, 2011), peptic ulcer (Matsuo et al., 2003), arthritis (Lotlikar et al., 2010) and asthma (Patel and Amin, 2011) are among the many conditions reported to follow distinct circadian patterns.

Cardiovascular diseases have been reported to predominate between 6 am and 12 pm. This is because of increase in cardiovascular activities such as rapid rise in BP, a rapid increase in sympathetic tone and concentrations of pressor hormones, and the highest values in peripheral resistance (Lemmer, 1996). It is thus easy to appreciate that pathophysiological events within the cardiovascular system do not occur at random (Willich and Muller, 1996).

Similarly, glucose rhythm shows a circadian variation, with its peak value in the early morning hours. This is because insulin levels and the hormones which work against insulin’s action are influenced by circadian rhythm (Suresh and Pandit, 2009). These hormones which include glucagon, epinephrine (adrenaline), cortisol, and growth hormones raise blood sugar levels at given times. For instance, during the middle of the night time hours, there is a surge in the amount of growth hormone released by the body, and is followed by a surge in cortisol (Suresh and Pandit, 2009).

In a non-diabetic person, these effects are annulled by a compensatory increase in insulin secretion by the functional pancreas and as a result, normoglycaemia is maintained. However, in type 1 diabetics where there is absolute insulin deficiency, and in type 2 diabetics where there is insulin resistance, changes in blood sugar levels during rest can have a deleterious effect on morning glucose levels. This is where the dawn phenomenon occurs, and glucose levels rise between 4 am and 8 am (Bolli and Gerich, 1984).

Furthermore, several studies have shown that the time of drug administration, especially with reference to biological or circadian rhythms, affects the kinetics and dynamics of various classes of medications (Lemmer et al., 1991; Hermida et al., 2007 and Hermida et al., 2008). Hence, for improved efficacy in drug therapy, it is important that drug delivery is timed to match the rhythms of disease conditions.

1.2 Statement of Research Problem

Persistent hyperglycemia has been shown to be a major determinant for the progression of diabetic microvascular complications (Somani et al., 2012; Bodhankar and Kamble, 2013). Adequate treatment is therefore required to maintain blood glucose levels, and this is coupled with treatment measures to delay the onset and prognosis of microvascular complications. This involves the use of glimepiride (Baynes, 2015), and nifedipine which has been recommended for microvascular complications especially nephropathy (J-MIND, 2001; ALLHAT, 2002; Padwel and Laupacis, 2004).

However, a deleterious drug interaction occurs when these two drugs are co-administered (Gillian, 2001; Suresha et al., 2012). This is because insulin secretion from pancreatic β-cells is tightly coupled to Ca2+ influx through the L-type Ca2+ channels (LTCCs) (Ashcroft et al., 1994). Dihydropyridines (e.g. nifedipine), a group of calcium channel blockers on the other hand reversibly bind to the LTCCs complex in various insulin secreting cell lines with high affinity (Yaney et al., 1991; Roenfeldt et al., 1992), and selectively block these channels (Catteral and Striessnig, 1992; Striessnig et al., 1993) and are thus potent blockers of insulin secretion (Yaney et al., 1991; Ashcroft et al., 1994; Davalli et al., 1996). This will ultimately impair glycaemic control and consequently exacerbate microvascular complications in type 2 diabetes (Bodhankar and Kamble, 2013).

1.3 Justification

Studies have shown that nifedipine delays the onset and progression of diabetic microvascular complications (Ravid et al., 1996; Estacio et al., 2000; Shigeaki, 2001 and Toyohiko et al., 2014). It has also been established that Ca2+ is required for the maintenance of normal oscillatory performance of the hypothalamic suprachiasmatic nuclei (Lundkvist et al., 2005). The use of nifedipine may affect the main body clock and other local clocks thereby altering major rhythms including glucose rhythm. Furthermore, by blocking calcium channels, nifedipine antagonizes the insulin secretory action of glimepiride (Gillian, 2001; Suresha et al., 2012). This will lead to reduction in glimepiride-mediated insulin secretion and diminish glycaemic control, with increased risk of microvascular complications in diabetes there by making it difficult to co administer the two drugs. Finding an alternate dosing or timing regimen by chronomodulation of the drugs, may offer an acceptable and effective alternative to concurrent administration. This may preserve the therapeutic benefit of both medications in diabetes with consequent improved glycaemic control and prognosis of microvascular complications.

1.4 Aim

To investigate the effect of time of administration of nifedipine on glycaemic control and microvascular complications in glimepiride-treated hyperglycemic rats.

1.5 Specific Objectives

The specific objectives of the study are:

1. To evaluate the effect of time of administration of nifedipine on glycaemic control in glimepiride-treated hyperglycaemic rats.

2. To investigate the effect of time of administration of nifedipine on the progression and prognosis of peripheral neuropathy in glimepiride-treated hyperglycaemic rats.

3. To determine the effect of time of administration of nifedipine on the prognosis of nephropathy in glimepiride-treated hyperglycaemic rats.

4. To evaluate the effect of time of administration of nifedipine on the prognosis of retinopathy in glimepiride-treated hyperglycaemic rats.

1.6 Hypothesis

The time of nifedipine administration affects glycaemic control, and exacerbates microvascular complications in glimepiride-treated hyperglycaemic rats.