I'm reading Option Volatility and Pricing by Sheldon Natenberg who in the chapter on Risk Management is trying to explain the effect of interest rates on options.
The value of a stock option will also depend on whether the trader has a long or short stock position. If a trader's option also includes a short stock position, he is effectively reducing the interest rate by the borrowing costs required to sell the stock short. This will reduce the forward price, thereby lowering the value of calls and raising the value of puts.
How can holding a short stock position affect the interest rate on the option?